Do you happen to know which format the drive was in?
Was it "Mac format" (HFS+)?
Or was it something else?
Do you have access to a Windows PC, by any chance?
What happens if you plug the drive into the Windows machine?
If it mounts in Windows, GET YOUR FILES OFF OF IT, onto some other storage media.
If it DOES NOT become accessible in Windows, DO NOT ATTEMPT TO DO ANYTHING MORE. Disconnect it for now.