1990 GMC Sierra won't turn over

Are you getting spark and fuel to the engine?

If you have good spark and the engine is getting fuel, it should run. The only other things that could cause it not to is timing or low compression. Are you getting a good hot blue snapping spark? Also, are you sure the injectors are getting power? You could have fuel to the throttle body, but the injectors may not be working. Will it start with starting fluid? Also, as far as timing, on the harmonic balancer there is a timing mark (hard to see if dirty). Align the mark on the harmonic balancer with the iming marks on the timing chain cover that says TDC. Next, note where the number 1 spark plug wire plugs into the distributor cap. Remove the cap and the rotor should be pointing to that spot (#1). Let me know what you find. Also, let me know if it starts with starting fluid.

By the way, Happy Easter.
Joe