The drive has been damaged when it fell off, most likely the read/write heads have crashed into the platters which contain your data. The drive is now unusable.
The only way to rescue any data which hasn't been damaged is to send the drive to a data recovery company, but that is very expensive, and they won't be able rescue all the data anyway because some of it has already been destroyed by the mechanical shock when it dropped.
You should never save important data on only one drive. It should be saved on at least two separate drives, because even if the drive is never dropped, it will still fail eventually just by the wear-and-tear of using it.