Hey there, NikkiAngela!
I'd suggest to try plugging the external in a different USB port on your Mac. You can even try with a different USB cable if you have a spare, just to see if it will get it recognized. Can you please tell does it show in Disk Utility on your Mac?
Here's a link that will explain how to get there:
http://products.wdc.com/support/kb.ashx?id=qRpTej
If you do see it (on the left), I'd suggest to select it and then click on Repair Disk (on the right).
Hope this was helpful! Keep me posted!
SuperSoph_WD