The accidental disconnect may have corrupted the disk.
Try a SMC reset:
Have you tried a Safe Boot?
Does is appear in Disk Utility?
Have you tried connecting it to another Mac?
Have you booted in Recovery Mode and see if it is recognized?
Thanks for the quick reply.
When I checked in Disk Utility, I got a completly different - and very simple - idea: I swaped the USB cord, this did the trick, the volume reapeared.
I hope the backup will work properly.