Check engine light back on

I have a 2008 ford escape xlt v6.  Check engine
light was on, the car was running rough. brought it
to my mechanic for a diagnostic check.  He
checked it and said ignition coils and spark plugs
need to be replaced. So he replaced all the ignition
coils and spark plugs. I  got my car back check
engine light was off.  Went home parked the car.  
Then two days later I went back to drive the car.  I
turned the key in the ignition and the check engine
light came back on.  I spent alot of money on the
repairs, this is very frustrating. What else could be
the problem.