If a 1976 Chevy Blazer with a 454 overheated due to the fan belt snapping now it will not start what could be some possible problems?

If it won't even turn over, then I would suspect your nonstock engine got severely overheated and warped or welded something important. Engine's gotta come out. If it turns over slowly, maybe the battery got run down because the alternator wasn't turning and therefore not charging.