During the events of the Bridge of Khazad-dûm, the Fellowship witness Gandalf facing the Balrog. When Gandalf falls, he's still hanging on the cliff. And when Frodo cries out to Gandalf, Boromir prevents Frodo from going to him. Why didn't the fellowship help Gandalf to get up?
In the novel, the events played out slightly differently. Gandalf was already at the rear of the pack. The Hobbits, Gimli and Legolas had already made it to the arch when the Balrog appeared. They stayed inside the arch (obeying Gandalf) whereas the humans, Aragorn and Boromir decided to stand with Gandalf.
'Over the bridge!' cried Gandalf, recalling his strength. `Fly! This is a foe beyond any of you. I must hold the narrow way. Fly! ' Aragorn and Boromir did not heed the command, but still held their ground, side by side, behind Gandalf at the far end of the bridge. The others halted just within the doorway at the hall's end, and turned, unable to leave their leader to face the enemy alone.
Gandalf seems to have taken a few steps forward to attack the Balrog and in doing so, he caused the bridge to break. His companions (who at that point were about 10-15 feet behind him) were forced to run away, lest they fall into the chasm.
When he actually fell, it was over within seconds.
With a bound the Balrog leaped full upon the bridge. Its whip whirled and hissed.
'He cannot stand alone! ' cried Aragorn suddenly and ran back along the bridge. ' Elendil! ' he shouted. 'I am with you, Gandalf!'
`Gondor! ' cried Boromir and leaped after him.
At that moment Gandalf lifted his staff, and crying aloud he smote the bridge before him. The staff broke asunder and fell from his hand. A blinding sheet of white flame sprang up. The bridge cracked. Right at the Balrog's feet it broke, and the stone upon which it stood crashed into the gulf, while the rest remained, poised, quivering like a tongue of rock thrust out into emptiness.
With a terrible cry the Balrog fell forward, and its shadow plunged down and vanished. But even as it fell it swung its whip, and the thongs lashed and curled about the wizard's knees, dragging him to the brink. He staggered and fell, grasped vainly at the stone, and slid into the abyss. 'Fly, you fools! ' he cried, and was gone.
The fires went out, and blank darkness fell. The Company stood rooted with horror staring into the pit. Even as Aragorn and Boromir came flying back, the rest of the bridge cracked and fell. With a cry Aragorn roused them.
Because Gandalf told them not to. When they first encountered the creature, Gandalf tells them
This is a foe beyond any of you. Run!
And then, when he is hanging on the bridge
Fly, you fools!
Remember, the orcs were still coming. Time was of the essence.
Since you are referring to the film, I think the best answer is the director's commentary from the writers Peter Jackson, Fran Walsh and Philippa Boyens.
They said that there are a few reasons in this scene. One is that they are actually far away from the bridge and that it had risps and could have been collapsed. Philippa also mentions that there is one scene in the book where Frodo tells Faramir that Aragorn and Boromir had to watch out for them and if they didn't had to, they would not have run away. Also Fran said that Gandalf with his quote "Fly, you fools!" meant that he let go rather than that he fell. Because he knew it was the decision he was going have to make after the moment he realised the mind of Saruman and his betrayal.