when it happened to me, it turned out the part of the hard drive that contained the actual windows software went corrupted.
if all you need is the data on that drive, a simple way would be to take out that drive and plug it into another computer as an external (or even an extra internal) drive. they sell drive enclosures that should allow you to do this. just make sure the computer doesn't try to boot from it or you'll just have the same problem.
of course a corrupted windows program is only 1 of the possibilities. the hard drive plug inside the laptop can be physically damaged (happened to my acer laptop. don't tell me yours is acer...?) worst case scenario: you better hope the drive itself isn't physically damaged...