Strangely, a good whack can actually temporarily fix a monitor, as can the ol' dropkick. It's just oxidized contacts and the occasional cap or something that gets out of line.
Another possibility is unplugging and replugging the monitor, and, if it has a detachable cable at the monitor end, make sure and do that too. Sometimes those contacts just get oxidized and quit transmitting.
If you're particularly attached to the monitor, it may be worth a trip to the shop. One of my favorite Trinitrons did this sort of thing and simply had a couple of conductors in the cable that were shot. A new cable, (with the connectors on the inside of the case in this instance) did the trick. If it's in need of a new tube or flyback transformer (it doesn't sound like a flyback issue, though), well, the repair bill will be more than the monitor is worth, probably.