Since you don't mention your skill level I'll just remind you that the first steps you should take is to get a hold of your data if you haven't got it already or had a backup in advance! Several things can cause the symptoms you've described, but as mentioned before; you need your data if you haven't got them already!
If you're comfortable working with other operating systems and live environments you should easily be able to grab your data, if not here is shortly how to get them. This only requires another computer, a local network and a cd-rom or a USB-drive.
1) Download Trinity Rescue Kit from the website (I'm not sure if I am allowed to link, however, you should be able to Google it pretty easily).
2) Burn the ISO to a CD-rom (or onto a USB-drive, Unetbootin is an easy way of doing this)
3) Boot the computer from whichever boot medium you've selected to use
4) Make sure both computers are connected to the same network, you should use wireless networking.
5) Choose the option to run a local Samba server
6) Open Explorer on the healthy computer and enter the IP address in the following format: "\\192.168.1.100", replace the IP in the example with the one of the "ill" computer. It should be displayed on screen. Google this is if my explanation is unclear.
I am no native speaker of english, so if I have totally underestimated your skills and this is child's play for you I am sorry, this is written with the best intentions!