1. The boot information on the hard disk is corrupted for some reason (that happened at some time after the last boot, and of course it's totally unclear why). Then startup repair from your Windows 7 install disk will fix it, until it happens again (if you don't tackle the cause).
2. There is a hardware issue with your disk or your motherboard. Then you replace the broken component.
More details, and possibly other suggestions, in the google results.