Your info is pretty vague, so this may or may not help. Below is a copy of a post I made on another forum some time back.
If it were me, I would remove the gearbox/differential from the mower. Try turning the clutch wheel now. If it turns, the problem is not in the chain case. If it doesn't, open it up and see what's wrong. There's nothing but two sprockets and a chain in there. I have seen the needle bearings siezed on the big sprocket before. Whatever it is, it should be obvious.
If the chain case works ok, try turning the hex shaft sticking out of the gearbox. If it does not turn, your problem is in there. I've probably rebuilt a hundred of these things. 90% of differential problems are because of stripped axle gears which will cause free-wheeling. A locked differential is not a common problem. If your problem is in there you will have to open it up and see what's locking it. I don't think anyone will be able to diagnose a locked differential with any certainty.