If it only happens once, chalk it up to the vagaries of modern electronics. If it starts happening more often, my experience is that the DVR is on its last legs. In fact I called DirecTV yesterday because our HR21 has been spontaneously rebooting fairly regularly for about two weeks now (the last straw came Wednesday night when it rebooted while we were watching TV; my wife said "You WILL call customer service tomorrow!" because she was fed up....thankfully our other DVR still works fine). The call took about half an hour and the rep had me turn it on and go through various steps; when they were unable to diagnose the problem, she said they'd just sent a replacement box via FedEx, second business day.
I went through this in May 2010 with another HR21 and the replacement process was painless, other than the nuisance of backing up recorded programs to DVDs (no big deal in and of itself except that you can only do it in realtime; in this case it's a pain because the box in question is in a different room from the DVD recorder so I have to disconnect it, take it downstairs, hook it up there, etc.).
There is a way to put the box in diagnostic mode to test the hard drive and other issues. Do a red button reset, and then when it gets to "Running receiver self-check" press SELECT to enter diagnostic mode. The menus are self-explanatory. One of the tests can take over 90 minutes, so only do this when you know you won't need the TV for a while. (In my case I did all this last weekend and the box passed all the tests, but the reboots have continued.)