Rear wheels locked up

I was driving and heard clanging sound after 30 seconds didn't hear the noise. Stopped to check under bonnet turned off engine checked under hood couldn't see anything. Started engine put in reverse nothing. So put in Drive still nothing. Both back wheels appear to be locked or seized when in Neutral both front wheels will move when turn the steering wheel from side to side. But rear wheels won't move even to allow us to push it either forward or backward. What do you think it could be?