Check to see if it shows up in Disk Utility. If it shows up with a string looking something like /dev/diski02s2 (that's an example) then the system is seeing the drive but not as a Mac volume. It's usually an indexing problem. Otherwise I'd check out the downloads section at:
http://scsc-online.com
and download the hard drive troubleshootiing book they have. It's gross overkill for what you've got but I'm sure the problem is probably in their, and it's a PDF tthat''s free anyway.
There are a lot of things you can do, like try it on another port, confirm all the connections are good, sway cables in case one's gone bad....just common sense stuff.
As a side note, if the external drive housing has feiled it's quite possible the drive inside it may be 100% intact.