recovering files after a hacker attack

hacker overtook a customers computer and apparently didn’t respond well to microsofts updates. therefore corrupting multiple dll’s after the customer called microsoft on it the dll issue was “resolved” now it seems the system is falling apart and further corrupting so in a last ditch effort I am trying to reclaim files tedious as it may be I am now caught in the endless restart loop (and the drive is not recognized by other systems)please get me out