I have a dell d610 and when I power up I get a hard drive not found message.

You hard drive has gone bad...

Try unplugging and opening it up... uplug the HD cords, and then plug them back in... nice a firm. Plug it in and fire it up.

(Leave it open when you boot... )

Did it work? If not, take a screwdrive and gently tap the case of the HD with the HANDLE while the PC is on.. I know it sounds like a bad idea, but it may be stuck, and I have had this work... if it starts spinning (you will hear it wind up) then hopefully you can get in long enough to BACKUP everything you want as fast as possible!!

Good luck