Go to Start> Run> Type "diskmgmt.msc"... This will open a Disk Management window.
In the bottom half of your screen you will Disk 1 (your local drive), do you see a Disk 2 or 3 dogmatism's any that would be this drive? You can probably tell by the size and if not you can unplug the unit and open the screen then plug it back in and go to the screen and see what changed if anything!
What color are the lights on the unit? Are they flashing in any type of sequence?
Do you hear any mechanical noise coming from the drive?
Did anything happen that lead up to this?
If the unit is beeping and you are not able to access it to update Firmware and it is not shown in Disk Management, the drive is most likely defective, and you may want to contact your local support for an RMA...But come back here with the results first!