3.8 engine cranks over, then jams up!

I decided to get my car ready for the summer, and when I cranked it over, I got it to turn, but then it stopped suddenly. I put a socket and hand bar on it to continue the rotation, but it wouldn't move. Reversed the direction, and it moved. Went back the other way, and it jammed up again. When I rotate a little back and forth, it sounds as if it is hitting something, in the one direction. I rotate it backwards, 180 degrees, it seems fine. When I try to start it, it rotates, but jams hard. I need to know what may be the cause, before I waste time on the wrong areas. I have heard it may be a timing chain broke and is jamming it up, hydro-locking, bad push rod, or broken valve clip, starter jamming or something in the flywheel teeth jamming the starter, not sure. I had it do this a couple times last summer when starting it, and it would stop, sound like a "thunk" noise, with kick back, or even a backfire noise as well. Been since October, when I had it on the road, and I don't remember the exact noises, and always had it start after it happened. Any ideas, I need to resolve it ASAP!