Mac Pro is damaged, won't show the disk.

Hey,
My mac won't start. Or no, it will, since I'm typing on it, but it shows this utility thing that gives me 4 options : use time machine and restore, install os x lion from scratch, access the web or open disk utility. at this point I get that my mac is f*** up. So after I'm done crying, I decide to re install lion. Problem is, it won't show my 500gb disk, it just shows a 450Mo disk that is locked. At the beginning, it showed me the partition on which I installed windows on, but after I restarted, it just shows the 450 Mo one.
I'm really terrified. I want to access the disk and save some of my files, half of my studies are on it (I know I'm stupid I didn't back up)
Help please ?

G'day and welcome to the forums.

Have you started with Command R to attempt recovery of the Lion partition? If no joy try and boot from the Snow Leopard disc by holding down 'C' after the chime, and if it boots go into Utilities > Disk Utility > Repair Disk and see what is reported. Sounds like the hard drive has failed and here you be with no backup? Did you make a USB thumb drive with Lion installed prior to hitting the Install button?

I formated the disk and installed Snow Leopard on it. Works fine now. But I've lost everything.
Thanks for your answer

Gat an external and use Time Machine in future, its saved my butt a couple of times