Need help! Car won’t start

I had the strangest thing happen to my car. I had a bad spark plug which was causing the car to misfire and backfire. I changed all plugs and started the car and it sounded perfect again. I then decided to take a spin around the block but before I could get to far the car shut down. It will not start! The computer is not reading at all. It’s not getting any power to give any readings. I have checked all of the fuses, checked relays. There is no power at the injectors. I’m at a lost. Does anyone have any ideas on what could possibly be the issue? Someone suggested that a bad map sensor could cause such and issue. I have replace the crank sensor and the cam sensor still no power no start. I even contacted PSI to inquire about the possibility that the computer has gone bad. But they say that these computers rarely go bad. But what do you guys think. That car had been running fine for years and no major issues or problems. I would like to get it back on the road. Hard to believe that a simple spark plug change could cause any issues. Like I said, after changing the plugs it started up just fine.As I drove it down the street it shut down.

dont go throwing parts at it yet, a couple things you mentioned sounds like your not getting power to the engine harness. first off, what is your setup, what engine, whos engine harness, what style computer, is it cable throttle or drive by wire?

you mentioned you have no power at the injectors, just for reference the computer doesnt send 12v to the injectors or coils, that comes directly from the fuse in the harness when the key is on. id check 12v on a number of things, see whats what. most aftermarket harnesses follow the gm coloring, a pink wire or pink with white is generally always switched 12v. check for 12v at your injectors, coils, mass air flow sensor, and 02 sensors. if all these things are dead, move back to the fuse block on the engine harness. you mentioned you checked fuses, how did you do this? i would check them with key on and a test light, they should be powered with the key on (maybe not the fuel pump fuse, depends on the harness). if those dont have power you have a problem with the relay that powers the harness, or with where switch 12v is coming from for the engine harness.

i would also check your ground connections, if the bolt holding the ground wires to the engine fell out it would produce the same sort of thing.

The engine is from a 98 Z28. I’m using a harness and a computer supplied by PSI Conversions. With the key on, there is no power going to the injectors. I did check the grounds. However, based on your input I’m going to check everything all over again. This is a drive by wire set up. PSI supplied pcm 1003 and harness for 1998-2002 GM LS1. HAR-1013.

I've had an intermittent issue with the immobilizer bypass done to my car back before I owned it. Apparently the original alarm systems were pretty well incorporated into the fuze box. There's some relay needed and often owners used cheap aftermarket ones that can cause troubles later on. Good luck.