I have had enough clunkers in my day and during the times that my cars have turned over but not started, it has been because the battery was running out. It had enough juice to turn over but not enough to start.

This is easy to test, just jump start it. It is probably more serious than this but it is so easy to test it is worth the time.

The one time I had an engine seize, you could not get it turn over. You could see and hear the starter engine going, but it was definitely frozen and would not turn over.