what is a sae j1850 pwm sensor for,on a 99 mer cougar 2.0 z-tec?

Question

I changed the head gasket now can't get fuel to fuel rail for some reason? I was told it was a electrical problem but why would that happen just changing the head gasket? I've done it thirty times if I did it once. Now all of a sudden It won't crank,it is getting fire to the plugs but won't fire off????? thank you very much james jarvis