My computer wont start up?

when i turn on the power on on my computer it runs but nothing shows up but a blank black screen. the last thing i did was a disk check on the sys diagnostics. it failed and i couldnt turn it off by pressing power as i had to take the battery out. then i turned it on and nothing came up. the last 2 disk checks that i ran failed and had to take the battery out just to reboot it but when i turned it back on the last 2 times even though it failed it worked just fine. why doesnt it work now and what can i do? i absolutly cant afford to replace my hard disk 1 because i have very important files i have to have.