disconnect the door and try it....the reduced "load" may "jumpstart" it then reconnect and try if it runs under no load. If it does not run under no load then it may be time to upgrade....How old is this?

Also could be if its a chain drive that the chain fell off sprocket and the motors running but not pulling on chain.

If motor is not spinning, then the unit most likely seized and its time to replace with new unit.

chamberlain, Liftmaster Sears, chain drives. have a plastic gear kit which sometimes will strip out. here's a link to how to repair it on youtube

http://www.youtube.com/watch?v=nmZlYQAZmLg&feature=mfu_in_order&list=UL