While I sympathize with the story the fact you installed an OS to a drive that had files on it is a very bad move.

What we do in such cases is to slip in a new blank HDD to bring the machine back then the old HDD is put into some case for access and retrieval for those folk that don't backup. Cost? about 70 bucks. Anyone that pushes back on that has told us their files are not worth 70 bucks.

Now it may cost hundreds to recover. Get an estimate from drivesavers.com
Bob