External hard disk fell on the floor

If the drive isn't working now then chances are that its suffered mechanical damage that can only be repaired by having it sent to a service with the clean room facilities to open the drive up and replace the damaged components. That will likely cost you at least a few hundred bucks up to thousands, depending on the extent of the damage.

You can try taking the hard drive out of the external enclosure, then plugging it directly into the computer via sata. If it still is not detected then yes, you have probably lost all your data. The data is still on the discs of the hard drive, but you can't read it unless the mechanics are repaired, which is not cheap or easy. Basically the little laser that reads the discs has to be EXTREMELY close, without touching, the hard discs, so when you bump it or drop it and the extremely delicate components are damaged, there is no easy fix for that.