Common problem, but zillions of possible causes. Try booting the recovery console from your windows CD to see if it can still see your windows installation on the disk, and then try the fixmbr, fixboot... You could try various tools on the HIREN CD to diagnose and/or fix the problem. But chances are you're going to be facing hours of frustrations. If the hardware is still OK, the easiest is probably to do a backup of your disk, recover from the previous backup and see if you're missing data.