Did you re-calibrate your radio to the esc when you got it back. I would:

Assuming your car is built right.

1. Re-set-up the ESC to the radio. (Make sure all connections are tight and you don't have any loose wires or solder connections)
2. Try a different battery
3. Try a different receiver/radio (if you have one)

If your esc is programmed to your radio and your car is built correctly, I would think this is an ESC problem.