Undo .. is a little tricky...
But: you can move your files back to a folder structure with the converter "Tag - FIlename" provided you have filled enough tags to create a folder structure.
If you select the files for moving and apply a mask that includes the backslash between the individual elements you will get a folder structure.
Depending on your preferences add the variables.
E.g. a structure with a folder for the artist, the albums and then the tracks per album would look like this:
%artist%\%album%$num(%track%,3) _ %title%
Of course, the mask is just an example.
If you want to sort by first letter of the artist's name then the mask could be extended like this:
$left(%artist%,1)\%artist%\%album%$num(%track%,3) _ %title%
Or if you want the YEAR as well in front of the album ...
%artist%\%year% - %album%$num(%track%,3) _ %title%
Or if your tracks should sort alright on any MP3 player that sorts by filename:
%artist%\%album%\%artist% _ %album% _ $num(%track%,3) _ %title%
The only thing that won't happen: if your previous folder structure was more or less handmade (which is more or less unstructured) then it will not be very likely that the structured converter approach will shift the files back into the old folders.
But you could take the opportunity and rename all files according to the new way based on tags.