If it was "working fine" and then the next time it "just wouldn't go", perhaps you have a problem with one of the clutches. Try blocking the rearend of the kart off the ground with the frontend up against a wall. Rev the engine up, and make sure the driver pulley halves squeeze together as the revs increase. Maybe it needs cleaning/lube. Perhhaps you have a seperate component TC like the Comet model 40 and something has loosened and shifted (like the motor mounts), causing the two pulleys to move closer together, effectively making the belt too long.
DO check out the specs and ID your TC and verify belt length, but- I very much doubt it stretched overnight from "fine" to "didn't go".
Cheers!
And PS- Kenny and Anderkart WERE trying to help you- but you gotta help us help you, by providing us with some info! When you didn't answer the questions but continue to ask for help, it's like a beggar standing there with his hands out and then refusing a job opportunity... AND, nowhere in your posts did you previously state that it was not the original belt! So- let's not get into a PI$$ING MATCH, and just try to have some fun!
-Chris