I assume your Mac is working its just not able to access the external WD drive.
OK open Disk Utility which is in your Utility folder in the Apps Folder. See if you can see the drive that way. If you can that would imply the drive needs repair.
If not you could have a bad connection to the drive (likely USB) see if you can access a USB thumb drive or other device via the USB ports of your system if you can then the cable to the drive or the drive its self is in trouble. If your system is not able to see other USB devices then you have a bad USB port. Try another.
If that too fails you will need to get a Powered Hub. Connect your HD to the hub (which has its own power supply & AC connection) then connect your Mac to the hub are you able to see the drive now? You may want to try another USB device off of the hub to verify things.
If this works then the fuse link within your systems USB port/s are blown. If you can access another device just not the HD then I think your drive is in big trouble.