Are other drives working just fine? If so, then it's the hard disk, the cable, or the filesystem. I'm thinking cable or drive, because if it was filesystem, your OS would let you know in an instant.
Try running WD's diagnostic against it - the SHORT test. Given what you're experiencing, it should fail quite quickly with an error code that can help you figure out what may be going on.
WD Support / Downloads / SATA & SAS / WD Caviar Green / GP
If the test passes, then run chkdsk against the drive to check the filesystem.
Don't do any "repairing" of the drive if you need to get data off it.