Lets start with the easiest (possible) solution:
Try using a known good USB cord and plug in with that.
Also, try all USB ports so we can eliminate that as a possible problem.
Can you fill out your system specs in the bottom left of your post OR in the USER CP? Also the make and model of the HD you are using.
Do not reformat. Worst case scenario we can get the files off your HD before you do that.
If you get it working, even for just a minute, make sure you backup everything while it's working.