WD External drive not recognized on computer

If the drive was running when it fell, and it doesn't have a head locking mechanism (or was in mid-read, or mid-write and the head wasn't locked down) then your disk may have experienced a head crash. The read/write heads float over the surface of the disk. If they slam into the disk, they will make a divot, boucne, make another divot, and continue bouncing until the kenetic energy is dissipated. In the places where magnetic material is removed, you won't get any data back. You may be able to recover data elsewhere, using a recovery company, but it won't be cheap. If your data is critical, you have a good chance of recovery using a service such as Seagate Recovery
http://seagatedatarecovery.com/ They recover data from all brands and I have used them They're very good, but very expensive ($800 range)