Well, there aren't too many things that should stop the crank from turning, but..., if you lost the relationship between the crank and cam, the only big item that comes to mind is a piston bumping up against a valve that's open. Nothing else makes any sense to me. At any rate, you will have to get the crank and cam back in synch. There is a mark on the front cam journal cap that lines up with the timing mark on the cam. That should be TDC for #1 cylinder on the compression stroke. However, other valves are in various stages of opening and closing so you may not be able to get the crank back to TDC for #1 w/o hitting valves as you are now. There are two ways you can go. Remove the crank damper and the front cover and manually install the chain. Remove the cam from the top of the head and then get the new chain around the crank sprocket, hold on to the chain, turn the crank until you are sure #1 is at TDC for compression, then reinstall the cam and realign the timing mark and reinstall the cam sprocket. You may me able to feed the chain around the crank sprocket w/o removing the front cover if you are lucky, but at any rate you will have to get all of the valves out of the way and that means cam off valves closed.
1987 BMW 325