Hi, i have just just fitted a new head gasket and serviced the car at the same time whilst also changing the oil sump.

I restarted the engine but now the engine warning light has come up, can anyone tell me what could have caused this.

Cheers

Chances are -- and you don't give us a lot of help by telling us what the car is -- that the timing is a tooth or two out or you didn't plug a sensor back in.