Did they put the timing belt in correctly? It could be out by one tooth. That would mess up lots of sensors, and the engine timing would be out. Or one of the sensors wasn't connected. Anything could be wrong really. Take it back to the mechanic and tell him/her (politically correct) to fix their mess. I would also look at the timing belt tensioner.
If that doesn't help, try and determine exactly what the symptoms are. EG is it only heavy on fuel, or is it not revving as high as usual. Hesitating to accelerate? Hard to start?
EML light off at first, but comes on later, or on from the start?
See what is different, and we try take it from there.
cheers