It may just need a drive letter assigning to it, as Windows sometimes fails to give ext hard drives one automatically. Go to
Control Panel > Admin Tools > Computer Management > Disk Management
In the bottom-right panel, look for the ribbon that relates to the drive (it will often say "Raw" in the description). If you see it, right-click on it and choose "Change Drive Letter & Paths".
Click "Add", choose a letter and click 'OK'
If the drive is not even shown in Disk Management, it may have failed. If testing software can detect it, it will tell you if the drive has failed. Test it with
WD Data LifeGuard for Windows:
http://support.wdc.com/product/download.asp?groupid=203&sid=3&lang=en