Try the above first ^
There could be a few things that I can think of, but whatever the situation...
Get your data off that drive asap!
#1. I have an external 250GB and for the longest time, I was unable to access it. The drive would not show up under windows and any partitioning software that I owned at the time would just tell me that the drive is unusable or couldn't be recognized. This went on until I reinstalled Windows 7. For some reason, unbeknownst to me, that solved the issue. I don't know how badly you want to do a reinstallation and I seriously doubt this is the issue, but it's an option before trashing the drive.
#2. Update the drivers. Yes I know there's really no hard drive drivers out there and it's really unheard of to go and update the drivers, but see if you can get some from the Manufacturers site and update them.
#3. You're harddrive is indeed failing
Get the data off and replace her.