Various things can cause this issue, but it should be rare.
One issue I've seen addressed is an apparent bug in the Amazon Kindle app that causes the screen to malfunction... The device is actually up and running, but since the screen stays off, it appears "dead." In this case, the issue will eventually "cure itself" when the battery finally dies, or you do what you did: power+UP volume to reset.
I was using the Kindle App quite a bit on Monday and left Kindle running when the Xoom went to "sleep." When I came back a few hours later, it was in the same state. It just wouldn't wake up.
Unfortunately for me, I thought the hard reset keybutton sequence was "Power + DOWN volume." I kept trying and trying and nothing would happen. Finally got online with my DroidX and discovered my memory had failed me. Power + UP volume reset the phone immediately, no problems.