The story involves the main character (I want to say a scientist) finding/inventing a way to time-travel. He goes back in time, and screws up (I forget the details.. Squishing a bug, perhaps, or leaving a pencil behind or something), and when he comes back, expecting a 'Sound of Thunder' kind of change... nothing has happened.
Wondering what is going on, he goes back and makes a larger change. Still nothing. He keeps going, escalating the nature of his changes (I think he kills his grandfather at one point), yet nothing changes. Then, on one return, he's unable to touch the controls.. After some frustration, he exclaims aloud and is answered...by another former time-traveler.
The other explains to him that common understanding of time & causality is flawed. Each person has their own chain of history, and they don't interact. He uses the analogy of a pot of spaghetti, with each person's personal past being a strand. Our hero, in making massive changes to his own past, has basically severed it, and he has come loose. He can now see OTHER strands (and those who are like him) having become, it is explained, the Spaghetti sauce.