Most likely the hard drive has failed. Power on, see if you can get into CMOS setup (F2). If you can, see if the hard drive is still detected.

If it is, restart, F12, Dell diagnostics. Check the drive.

If it's not, reseat the cables, try again. Still not detected?

Under warranty: call Dell for replacement.

Out of warranty: buy a new drive of the same type (SATA or EIDE). Avoid Maxtor.

Then replace the drive, boot the OS CD, reload XP, patch fully, reload apps, patch, and finally restore your data backup.