Dave, nearly all tillers are chain drive.
There's an oval cover or plate on one side of the machine, where the chain hides.
There is often a sight glass for oil level on it.
If you disconnect the chain, usually by removing a drive sprocket, you can then determine if the gearbox is locked, or the tiller side bearings are seized.
It may well be that the chain itself is the problem, from lack of lubrication.
Just a thought, make sure the tiller tines aren't stuck in the soil or something wedged back under the tines and frame...the back usually swivels up to regulate the amount of soil pulverization,and mine sometimes hangs up on a tine.