There could be a couple causes for this:
* The hard drive is not connected or is unmounted / ejected
* The hard drive is corrupted - run Disk Utility disk verification checks to make sure that this is not the case.
Or like mayer said, the drive may need to be formatted. If you already have data on this, you should definitely check the above ideas first so as to not lose vital data.
I always verify my disk in Disk Utility every couple months to make sure that the disk isn't corrupted or failing. My time machine disk corrupted once on me, and I lost all of my backups (I found out when my time machine disk failed to back up my hard drive one day).