I turned it on last night and it clicked right off and it won't stay on. What could cause this? It worked fine just 30 minutes earlier.

Maybe the speakers are drawing too much current? Or a bad power supply...

Does it stay on without anything else hooked to it?

I've had the same setup for a quite some time now. How could any of that change? Anyway, as of this post it's at least staying on for now. So perhaps some of the stuff you mention could be happening.

Check for a frayed speaker wire connection or damaged wire first.

Have you moved the receiver recently? You speaker wires may have come in contact with each other shut it down. I just recently had a friend call me over because she would cut the receiver on and sometime it would stay on and others it would shut down after a few seconds. When I pulled it out whoever set it up had the exposed positive speaker wire to long which was creating a connection with the negative terminal. Just a suggestion.

This is more than likely it. Even if it looks ok, redo the wiring on the receiver end. I used to have a lot of problems with this when I first got it. I'm terrible with copper wires.

If you don't have them already, pick these up - they eliminate the problem entirely.

Assuming that actually is the problem.

I have seen this happen because of a cracked cold solder joint, a dust bunny that has moved or simply an overheat situation.

Thanks for the tips guys. I'll look back there and do maintenance.

EDITED TO ADD:

Dangit!! Now that it's working and the problem so far sounds like a simple one...I'm kinda bummed. Hell, I was already lookin' online so I could upgrade about 10 minutes after the problem started.

Had the EXACT same problem with my Yahmy receiver the other month when I connected my Xbox 360 to my receiver. Here's my post and the suggestions given: http://www.dvdtalk.com/forum/showthread.php?t=449884

The problem turned out being a few speaker wires barely touching another speaker wire or the outside shell of the receiver (couldn't tell which exactly). Just rewrapped the wire around the +/- posts and taped up my banana plugs that had part of the plastic sheathing splitting. That solved the problem.

People always seem to cut off too much insulation and leave a long bit of wire showing. Really makes it more possible for that to happen. I confess to the same problem. I have a couple that are just waiting to do this to me. Have I fixed them?......