In the book, The Fellowship of the Ring (chapter 12 "Flight to the Ford"), all nine Nazgûl were swept away by the waters of the river Bruinen, and their horses drowned. They show up later in The Return of the King (chapter 6, "The Battle of the Pelennor Fields").
What did they do between those events?
Short answer: They walked home, unclad.
The last thing Frodo sees at the ford is
At that moment there came a roaring and a rushing: a noise of loud waters rolling many stones. Dimly Frodo saw the river below him rise, and down along its course there came a plumed cavalry of waves. White flames seemed to Frodo to flicker on their crests and he half fancied that he saw amid the water white riders upon white horses with frothing manes. The three Riders that were still in the midst of the Ford were overwhelmed: they disappeared, buried suddenly under angry foam. Those that were behind drew back in dismay.
With his last failing senses Frodo heard cries, and it seemed to him that he saw, beyond the Riders that hesitated on the shore, a shining figure of white light; and behind it ran small shadowy forms waving flames, that flared red in the grey mist that was falling over the world.
The black horses were filled with madness, and leaping forward in terror they bore their riders into the rushing flood. Their piercing cries were drowned in the roaring of the river as it carried them away. Then Frodo felt himself falling, and the roaring and confusion seemed to rise and engulf him together with his enemies. He heard and saw no more.
The riders fear the water. But what happened next?
Gandalf: Caught between fire and water, and seeing an Elf-lord revealed in his wrath, they were dismayed, and their horses were stricken with madness. Three were carried away by the first assault of the flood; the others were now hurled into the water by their horses and overwhelmed.'
'And is that the end of the Black Riders?' asked Frodo.
'No,' said Gandalf. 'Their horses must have perished, and without them they are crippled. But the Ringwraiths themselves cannot be so easily destroyed.
Later:
'I thought they were all destroyed in the flood,' said Merry.
'You cannot destroy Ringwraiths like that,' said Gandalf. 'The power of their master is in them, and they stand or fall by him. We hope that they were all unhorsed and unmasked, and so made for a while less dangerous; but we must find out for certain.
Still later, after Elrond sent out searchers:
In no region had the messengers discovered any signs or tidings of the Riders or other servants of the Enemy. Even from the Eagles of the Misty Mountains they had learned no fresh news.... Three of the black horses had been found at once drowned in the flooded Ford. On the rocks of the rapids below it searchers discovered the bodies of five more, and also a long black cloak, slashed and tattered. Of the Black Riders no other trace was to be seen, and nowhere was their presence to be felt. It seemed that they had vanished from the North.
'Eight out of the Nine are accounted for at least,' said Gandalf. 'It is rash to be too sure, yet I think that we may hope now that the Ringwraiths were scattered, and have been obliged to return as best they could to their Master in Mordor, empty and shapeless.
Why could they not steal more horses so they could ride? Their horses are not ordinary horses, but have been trained to endure the Nazgûl:
Frodo: 'Then why do these black horses endure such riders? All other animals are terrified when they draw near, even the elf-horse of Glorfindel. The dogs howl and the geese scream at them.'
Gandalf: 'Because these horses are born and bred to the service of the Dark Lord in Mordor. Not all his servants and chattels are wraiths!
Bottom line:
At least eight (and probably all) of the Nazgûl lost their horses and probably their equipment in the flood at the ford.
They feared "an Elf-lord revealed in his wrath", and knew that there were many of that power in Rivendell. They would not have lingered or even spent time dragging the river for their swords.
They themselves would not have been destroyed or probably even injured since "The power of their master is in them".
The would have been "obliged to return as best they could to their Master in Mordor, empty and shapeless." Since getting new horses was impossible and since they couldn't fly on their own, this meant walking or, conceivably, riding on a raft.
It's not even clear that they could find each other again — they would have been hiding as long as they were near Rivendell — no uncanny screeches on the wind!
It would probably take them at least half as long as it took the Fellowship (ignoring vacations in Lórien) to get back South since they might be able to travel night and day — there's never a hint that they need sleep — but it's highly unlikely they could have done better than twice the Fellowship's speed, so they probably reached Mordor no earlier than when the Fellowship reached Moria, and perhaps much later.
The Fellowship encounters the flying Nazgûl for the first time roughly a week after they leave Lórien, leaving no more than five weeks between the earliest time they could have gotten back to Mordor and their sighting over the Anduin.
Given the speed of their reptilian mounts, they could have gotten from Minis Morgul to the Anduin in a day, so they spent somewhere between a week and five weeks:
The Nine returned to their master and drew strength again, before being sent out on their new winged reptilian mounts. They stay hidden on the eastern side of the Anduin for several months, until after the Breaking of the Fellowship on February 26, 3019.
‘Nazgûl, Nazgûl,’ said Grishnákh, shivering and licking his lips, as if the word had a foul taste that he savoured painfully. ‘You speak of what is deep beyond the reach of your muddy dreams, Uglúk,’ he said. ‘Nazgûl! Ah! All that they make out! One day you'll wish that you had not said that. Ape!’ he snarled fiercely. ‘You ought to know that they’re the apple of the Great Eye. But the winged Nazgûl: not yet, not yet. He won’t let them show themselves across the Great River yet, not too soon. They’re for the War — and other purposes.’
Legolas shoots down a flying shadow to the east of the river near Sarn Gebir on February 23.
[A] dark shape, like a cloud and yet not a cloud, for it moved far more swiftly, came out of the blackness in the South, and sped towards the Company, blotting out all light as it approached. Soon it appeared as a great winged creature, blacker than the pits in the night.
...
Suddenly the great bow of Lórien sang. Shrill went the arrow from the elven-string. Frodo looked up. Almost above him the winged shape swerved. There was a harsh croaking scream, as it fell out of the air, vanishing down into the gloom of the eastern shore. The sky was clean again. There was a tumult of many voices far away, cursing and wailing in the darkness, and then silence. Neither shaft nor cry came again from the east that night.
Gandalf later confirms that there was a Nazgûl mounted on the creature Legolas killed:
'The Winged Messenger!' cried Legolas. 'I shot at him with the bow of Galadriel above Sarn Gebir, and I felled him from the sky. He filled us all with fear. What new terror is this?'
'One that you cannot slay with arrows,' said Gandalf. 'You only slew his steed. It was a good deed; but the Rider was soon horsed again. For he was a Nazgûl, one of the Nine, who ride now upon winged steeds.
One (or possibly more) Ringwraiths probably overfly Frodo, Sam, and Gollum while they are crossing the Dead Marshes on March 1–2.
The next appearance of the winged Nazgûl riding their fell beasts is probably on February 27, although Grishnákh indicates that the Rider has still not yet crossed the river, but has sent Grishnákh's contingent of Mordor orcs west to acquire the hobbits that were taken by Saruman's Uruk-hai.
'I came across,' said the evil voice. 'A winged Nazgûl awaits us northward on the east-bank.'
The orcs never make it back to deliver the hobbits to the Nazgûl; they are slain by the Riders of Rohan. After the destruction of Isengard by the Ents, Pippin looks into the Palantír of Orthanc on March 5 and sees Sauron himself. Sauron, believing that Saruman (who had possessed the seeing stone) had a halfling captive in Orthanc, sends a flying Nazgûl to retrieve the captive, and its passing overhead is dimly sensed by the heroes camped at Dol Baran in Rohan shortly thereafter.
As to the other details of their doings, the only authoritative source for what the Nazgûl were doing when they were not "on-page" (as it were) would be the timeline from the end of The Return of the King. However, there are actually no other mentions of Ringwraith activity between their defeat at the ford and the departure of Sauron's first army from Minas Morgul on the Dawnless Day (March 10), which Frodo and Sam see the Lord of the Nazgûl leading. From this point on, until the Battle of the Pelennor Fields, all of the Nine may be with the army.
