Why won't your car start after replacing the head gaskets?
With out a make model and year of the engine I am guessing.
First, make sure the cam(s) are timed correctly. If there a
distributor in this car, make sure it is timed correctly also. I
seem to always get this in wrong at first.
Second, do some basic checks, like Compression; Spark; Fuel.
Third, check to see if the plastic was bad on the Car, inside
the air cleaner.
Fourth, my all time favorite thing to ask myself is whether I
remembered to take the rags out of the intake ports.