Dual Monitors: was working but isn't anymore

[88jayhawks]

I use my lap top for everything and decided to hook up a second monitor. It worked great for a while but now, for some reason, it doesn't work. When I go under properties to set it as a dual monitor and click "apply", my lap top screen goes to a blank black screen and freezes. This happens for both XP and Vista where as it used to work for both OS'. When I restart my lap top, the second shows activity but then goes blank and a "no signal" sign appears on the second monitor.

Anyone have similar problems and any suggestions to fix this?

Thanks