It usually means that part of the hard-drive, where the files that instruct the computer to boot are stored, has been damaged in some way. This can happen if the hard-drive is getting old, or if the laptop has been exposed to heat/magnets etc; as my OH says, these things just happen, sometimes
Reinstalling your Operating System is usually the best way of sorting it out; did you get given the Windows disks and authentication key when you bought the laptop? If so, pop the disk in and reboot the laptop - you should get a set of instructions to help you *rebuild* the laptop - but you will lose any files stored and will need to reinstall any of the programmes (including any Windows Office applications like Word and Excel).
If not, you may be able to get hold of a copy of the missing file off the web, or through an independent PC engineer - if you put the file on a floppy disk and then boot the laptop up in "safe mode with prompt" then you may be able to get it to boot this way.
HTH