It sounds like the fall might have damaged the internal components of the drive.
🙁
If the drive is physically damaged I'm afraid there is very little we can do to help you. You can try to run a scan on the drive using WD Data Lifeguard, but if the drive is not even being recognized by your system this isn't an option.
http://support.wdc.com/downloads.aspx
If you had critical data on your drive you might have to seek the services of a 3rd party data recovery company. They are rather expensive though, so the data would have to be quite valuable to justify the cost.
Here is a link to the WD data recovery site, which lists all official WD Data Recovery partners: http://support.wdc.com/Warranty/dataRecovery.aspx