1. Check to make sure it is plugged into the electrical outlet and verify that all connections are secure.
2. Is the drive operating, lights working properly and do you hear a sound coming from the drive case? If yes go to the next step. If not skip to step 7.
3. Restart the computer. See if the drive is listed.
4. If the drive is not listed in My Computer then click on start and search for Computer management. Go to disk management and see if the drive is listed there. If you are using XP use this to get to computer management: http://support.microsoft.com/kb/308423
5. If you do see it & it requires initialization read this: http://answers.microsoft.com/en-us/windows/forum/window...
6. Try recovery software like recova: www.piriform.com/recuva.
7. (Note: Only do this if you are familiar with electronic safety & all precautions. Otherwise i would find someone you know that is experienced) If the above didn't work you could try opening the enclosure which isn't recommended if your drive is still under warranty. If you can open it up you can put this disk into your computer and see if it runs. It will be kinda tough to hear whether the drive runs depending if your PC makes noise in general. If you are familiar with the noise your PC makes when booting you might notice whether the drive you hooked up spins up as sometimes certain hard drives make unique sounds when operating (mostly a brand thing).
See if my computer detects it and if so then it's possible your enclosure went bad. If the drive doesn't appear to be working (refuses to spin up) then it might be the circuit board going bad. If it does spin up but makes a clicking or buzzing noise then your hard drive is no good and the only way you may get your data off is to send it into a recovery data company which is too expensive. If you do manage to get it working you'll need to get whatever important data off of it and while it might work if you feel up to it and could use a drive just to use as a storage (but don't store any critical data on it without backing it up first).
I have music and video stored on a 2TB drive but i use older drives just to play it thus using my 2TB as a backup device.