1986 Cadillac Brougham engine will not turn over

I cranked my car and put it in reverse to pull off. Car cut off which it usually does when not warm. Put car in park and attempted to restart car. Car would not turn over. Upon further inspection the starter was not disengaging. Took starter to have rebuilt and it was told nothing was wrong with the starter. Still had the solenoid replaced. Put starter on and the motor would not turn over. The starter would hit but not turn. So I was able to turn the flywheel by hand just to make sure the motor was not locked up. I need help with diagnosing the cause instead of having to replace main bearings.