It may be that Recuva isn’t able to read the drive for some reason - What filesystem did you reformat it to?
It’s always possible for a drive to fail, have you run ‘chkdsk’ on the reformatted drive?
However are you sure that you are using Recuva correctly?
I’m a bit concerned that when trying to recover files from an accidental reformat you say:-
You should not usually be scanning an accidentaly formatted drive for “deleted” files.
Ususally what you want back from an accidental format are the files that hadn’t bee deleted, the ‘live’ files that were ‘lost’ in the format. (Lost not deleted).
Follow the instructions in the link that I gave above, here it is again.
There is even a link to a video that shows someone doing it, have a watch of that so you can see what should be happening with Recuva.