I'm sorry to hear that you have troubles accessing your external WD HDD.
There are various reason for a drive to stop working and probably the most common one is the drive is not getting enough power. To make sure this is not the case make sure you plug the drive directly into the USB port of the computer and not through any USB hubs. Try using a different known good USB cable. To rule out low power output from the USB port try using different USB ports and different PCs if possible. You should also run WD Data Lifeguard Diagnostics to scan the HDD health. You can download it here: http://products.wdc.com/support/kb.ashx?id=M8IDQA. Keep us posted.