Ditto, Deb W - what kind of hub is it and how old is it?

My husband had a similiar problem with the front hub on his Trek 520 (he'd bought it off some guy and it was fairly old and well-used). The hub bearings were old and they totally locked up. The wheel came to a sharp halt. No budging it. Luckily, he was just starting out from a stop when it happened or he could have gone over the bars! He took it apart and regreased the hubs and it solved the problem. They were almost dry!

I would think that if the same thing happened to a rear wheel, the cassette would still be able to rotate in reverse even though the wheel wouldn't move.

Just a guess though...