Hey there, huckapril!
I suggest you try the drive on another computer to see if it gets recognized.
Also it's important to know how is your external HDD formatted - NTFS/HFS+/Mac OS Extended (Journaled)/FAT32 etc.?
Open up Disk Utility (Applications->Utilities->Disk Utility) and see if the drive is showing up there.
If it does, you can simply choose the option to Repair the drive.
If you cannot access it in any way on your Mac, it's most probably formatted as NTFS or some other not recognizable by Mac OS X file system.
Here's a link that can help you partition and format your external in any OS:
http://wdc.custhelp.com/app/answers/detail/a_id/3865
Hope this helps.
SuperSoph_WD