Faulty Starter or Siezed Engine?

I have a problem. My 1995 Lumina will not start. I had just gotten home and the car had been running fine. When I got back in to start it a couple of minutes later it seemed like the starter was working but once the cylinders started to fire there was a clicking/grinding sound and the engine seemed very unstable and it stalled when I pressed further on the gas pedal. I tried again a few times with the same results. Finally, the starter will now only emit a click when the key is turned and the engine doesn't turn at all. The battery is fine and fully charged. I tried to turn the crankshaft manually with a socket and breaker bar and I can't make the engine turn at all. Is it possible that the starter is siezed up and engaging the flywheel - is that why I can't turn the engine by hand? Or did my engine spontaneously seize up after it was running so finely just a few minutes before and the starter just can't budge the flywheel? Any input would be greatly appreciated....Thank you.

Timing belt could have broken. Yes something might have happened with the starter, but you should have been able to turn it over with a breaker bar. Pull it off, and if thats not it, check the timing belt. If thats not it you probably have something bad, like siezed bearing, broken or damaged pushrod, or something in that area. good luck.

The first thing I would check for is corrosion, both at the battery terminals and at the solenoid. Also check and make sure you are in park or neutral. Sometimes the switch doesn't make contact when you put in park and you have to drop it to another gear and back again.