Well, try doing this:
Go into BIOS by pressing 'Del' at bootup.
Now, in one of the menus (I think its the first) they'll be an option about "First Boot Device". Make the hard drive load first with Windows on as that, and then Save And Exit from the BIOS menu.
Hopefully then it'll work and find it.
If not, it could be possible your hard drive could have a fault, and i'd get on there and back up as quickly as possible