File rcvry, master sys admin NameChange

I have a windows 2000 system in which the System generated administrator name and password were changed.

While the change was in place files were saved, moved & position of files taken.

The individual then changed the system administrator back to it’s original name and password. Of course all files which the system administrator owned and those which were created and position was taken are not accessible.

how can the affected files be recovered? (system administrator as owner of affected files)