Timing chain jumps usually occur at startup or shut off, when oil pressure to the tensioner is the lowest.

Combining the worst of both worlds (no pressure to the tensioner AND the engine ceasing to move), your chain probably jumped. You can't move it backwards because the cam is trying to open the valve in that direction (it's closing when you move it forward), but can't move it forward because the piston is kissing the valve.

This is just the "Most likely" scenario. Your engine moved backwards a teeny bit due to compression and the chain moved on the cam. Try moving the cam on the right side of the engine first. A bit counterclockwise (AGAINST its normal direction of travel); MAKE SURE to mark its spot first. If it doesn't budge then move it 1 link clockwise. If it does, then try the other one.

Note: It could be the other way around but I would think it's less likely (that your valve is opening, because you can't seem to budge the engine backwards)