I have come home from holidays and when I powered up my hard drive the light continually is flashing and my computer is not picking it up. I have treid rebooting the computer and turning the hard drive on and off.
Check the drive on a different computer or with a different USB cable.
For other suggestions, please see the link bellow:
WD external drive is not assigned a drive letter by Windows or Mac OSX and data on the drive is inaccessible