It might have just been a minor power surge that prevented it from booting normally. If so it should start normally in future.
If it doesn't, trying booting it into Safe Mode (by pressing F8 while it's starting up). If it boots into Safe Mode, close it down as normal then leave it for a minute or so and try rebooting. If the problem reoccurs, close it down and then boot it into Safe Mode again, but this time run System Restore (from Start>All Programs>Accessories) to take the registry back to an earlier date.
If you can't get into Safe Mode you might have a serious problem. Please post again (while praying that Chuck is about!)
Chris