|Can't really answer that w/o knowing if your hard drive is toast. Could very well just be a Windows problem and no, your files would be fine. |
However, if it is a disk-related problem, then yes, your files could be gone.
I would boot to a live CD & see if you can access any files & copy them to another machine (before sending your computer in for service).
System rescue CD is my favorite for this task -
You could also try Knoppix or the Ubuntu live cd