Apple never sold external SSDs.
Is this a drive taken from a laptop, now in an external enclosure?
It could be the enclosure.
It could be the cable.
It could be the drive itself.
Or... it could be "a combination of factors".
If it's just a backup -- and if the "source data" on the backup exists elsewhere -- you haven't really lost anything.
In my experience, sometimes SSD's just "go dark" on you -- disappear, never to be revived again.