Password protection is one reason, hardware encryption is another. Does the drive show up in BIOS and Device Manager?
Inside the enclosure, is there an Initio INIC-1607E chip on the USB-SATA bridge board? If so, then this chip is responsible for hardware encryption, in which case you will not be able to access your data, even with the correct password.
Be advised that many users are reporting problems with the USB connector at WD's forums, so this would be the first thing to check. Otherwise, simply swapping the bridge board with another external WD drive will work.