I've found answers to this problem in the forum, but because I'm not using a WIFI Time Machine (i.e. not apple) the airport utility reset option is not available.

Anyway, I have a HD backup that connected via USB. Was working fine, but not am getting the error message "disk could not be mounted". Now, the disk does not show up anywhere. Not in finder, not in time machine preferences, so i can't actually get to it.

(This *may be related to the back up accidently being shut off, but i can't be certain)

Any ideas?