Bicycle Tutor

I purchased an old 18 speed recently, I gave it a vigourus test drive and other then a worn front tire and back brake I've seen no problems.
Anyway just a few days ago I started riding when a heard a clunk and the pedals lost all resistance and the rear wheel no longer spins when I pedal. I assume what has happen is the rear gear as been detached from something on the rear wheel or a bolt has slipped. I don't have much idea when it comes to bikes. Any ideas? I can take photos if required.

Do the sprockets move? If yes, then either the freewheel is broken, or maybe the ratchets (the part that allows you to coast but engages when you are pedaling) are just gummed up. You could try freeing them with a liberal amount of penetrating oil.
Look at http://forums.bicycletutor.com/topic/812 for more hints