2001 Accent engine light

I just had the 60k service done and after the car did not have alot of go.So I took it back were I had the work done a Hyundai dealer.And they said nothing is wrong , if the timming belt was off one tooth the car would not run.But now the engine light is on. Do I dare bring it back to them and tell me nothing is wrong again?!The car seems to run fine .The gas cap is tight , oil is good, coolant good?

The check engine light, along with the enemic performance, are pretty sure signs that something is wrong.  I don't think it's directly related to the timing belt, but more likely a part or sensor failure with the fuel system.  The CEL indicates an error code has been stored in the computer.  You either need to take this to the dealer (since the power train warranty should still cover the repair) to have them scan the code, or obtain a scanner and extract the code yourself.  Some parts stores loan scanners.  This step is all part of diagnosing the problem.