Urgent help requested – Machine wont boot and PHD thesis is stuck on it!

I have a dell dimension 8400, it has been working ok until today. When I start it up I get the black screen that tells me to either start in safe mode, last known config or start normally. I have tried everything and nothing works.

I thought I could boot from CD but no luck. I have checked in BIOS and the onboard or USB CD ROM drive is at boot sequence 1 but i noticed that at number 4 it says. ONboard IDE Hard Drive (not present) – what is this? is it serious..

Can any one help. I am due to hand in my PHD on tuesday and it is on this machine. Please