What is wrong when the check engine light comes on?????

I just had a new fuel pump and fuel filter replaced on my 2005 Chevy Malibu after the fuel pump went out coming home from vacation and 4 hours from my home. The gas tank had to be dropped and the tank was full of bad gas. They emptied and clean the tank, replaced the fuel pump and fuel filter and cleaned the injectors. On the way home from the repair shop the check engine light came on? What now? I check the gas cap lid and tightened it. Help???

could be a ton of things, from oil change, to o2 sensor, to whatever.....go get it scanned at a dealer or garage