PC suddenly stopped recognizing My Book Essential

First, make sure to test the drive on a different computer, cable or port.

Check if the drive appears on Disk Management and see what is the status.

Try the suggestions provided on the link bellow:

WD external drive is not assigned a drive letter by Windows or Mac OSX and data on the drive is inaccessible

http://wdc.custhelp.com/app/answers/detail/a_id/1164