No backup.....good grief.
Is this an external USB, Firewire or eSATA drive? Is it SATA or PATA inside? Model # if you don't know.
When you boot up your computer from another drive with OS X and open Disk Utilities, can you see the drive?
If nothing works, you might consider two options:
A Ubuntu live CD might be able to see the drive, you can run it from the CD drive OR
Removing the drive from the drive case and connecting it internally might improve matters.
Well, there are two kinds of people in the world: those that have lost data and those that will. You should consider an external USB Time Machine backup....and periodically restore a file or two to confirm that the backup drive is functioning properly and the restored files are intact. One should not make the mistake of assuming an untested backup drive is still functioning for long periods of time without validation.
