It's probably obvious to you already, but I'll say it anyway, it could just be a screen saver. If you wiggle the mouse or hit a key on the keyboard and your screen comes back, that's it. You can adjust the screen saver behavior in the Display portion of Control Panel.
If you have a laptop, it's possible that the computer has gone into standby or hibernate. If you simply push the power button and it comes back up to where you were (possibly after a few seconds), that's probably it. Normally a computer should only go into standby if you're not actively using it, but if your battery is very low, it my force standby in order to avoid losing what you're doing.