What it sounds like from your description:

When the computer power goes down (in stasis, sleep, hibernate, or most other forms) there's no signal coming from the computer. The monitor can't tell that it's not receiving a signal because the computer isn't sending any signal, so it guesses that the computer went next door and is out of range.

(Let's guess that it's a wireless monitor? Hard-wired ones usually just say "no signal.")

If this is what's happening, when you bring the computer back up the signal from computer to monitor will also come back, and everything should be just as before. You should be able to "wake up" the computer with some key or another, without a reboot.

IFF on the other hand you mean that after you wake up the computer you're getting the error message, it may take a moment or two for the monitor to figure out what driver settings it's receiving so it can ask for what it wants. If it doesn't settle in fairly quickly though, it probably means that you need an update for the monitor's driver. The computer may be substituting a default driver (Vista was pretty good at guessing wrong with the defaults) that doesn't offer the resolution and scan rate that the monitor wants.

Vista did have a tendency to flag you with notices that "your computer's performance can be improved" by changing things, but it always told me to change it to what it was already set up to. I never did quite figure that one out.

John