Freewheel sprocket locking up
Freewheels do require lubrication sometimes.

I suspect that if you drip some oil (I've used air tool oil, which is pretty light, but anything will be OK) in there with the bike tilted so the is freewheel up the pawls with become unstuck and it will work fine.

In my experience it won't take much oil or time, it should free right up if lubrication actually is the issue.