In most of the cases we see here, something got corrupted and a tool such as EASEUS Partition Recovery or any of the dozen others mentioned here will have a good chance of recovering your data. But if the actual data blocks have been overwritten, by new data or an OS install, it may be unrecoverable.
Can you provide a little more information about what happened? Did the drive contain readable data at one point? Can you see the data on some other machine?
Also, most external drives do not require "software to run it," they are plug-and-play. Is this a drive used for backup that had backup software on it, or what software was on it? Every little bit of information helps.