If the open one is still on the track, it may stay up if you can get it get back up and pull the fuse the windows are on. If it's partially off the track (one of the rollers has popped out) or the mechanism and / or motor are really shot, you might have to pull off the door panel and rig something up to keep it in place until you can fix it or have it fixed. The wooden shim mikes suggested sounds like a good idea. Once you've got the door panel off to access the mechanism, though, you're almost half way to fixing it. Shop around for the parts, if you can afford them, fixing it might actually be the easiest way. If you need cheap parts, see if you have a Pick-N-Pull or other 'pull it yourself' boneyard near you. In addition to discount parts, you get to practice taking the door panel off. Worst case, you can always tape up a sheet of plastic. You wouldn't be the first to drive around with one of those until you can afford a window fix.
See moving a dead power window in order to remove it for a description of one type of mechanism. Here is a video showing the removal of a cable driven mechanism.