Hmm...I'm not to sure why it would have happened, but I see why you would be concerned. I would keep an eye on it over the next couple of days to see if anything else strange happens, and maybe take note of what happened and when. Otherwise, a system restore has been known to clear up a variety of issues, and could very well have resolved this.
I don't think it was anything like a virus, as a virus tends to be more malicious and major than something like this. It is more likely you hit the wrong key combination, or a program failed to open or close properly. If you are really concerned, you may want to run a virus scan to make sure nothing is hiding out on your machine.
