Has all the data disappeared off my old core2duo iMac?

I have this old computer as a living room backup. It wasn't started for a while and then started booting to a shut down when just past the halfway mark on the blue ribbon progress bar. First aid wouldn't complete its verification and nor would fsck in single user mode.
Eventually I managed to get FSCK -Rc to repair a huge number of clusters (?)  and now the whole drive appears to be completely empty in DU
But First Aid runs fine.
I dont remember what OS was on it (it may have been Mojave) but trying to reinstall seems to want to run El Capitain.
Whatever it was, I can't re-install anything. At first it says drive locked and then it wants to do a fresh install of El Capitain.
What is the best way of getting answer to the question of what is on the drive or if DU says NOTHING, is that the only answer? I'd like to look to see if there were any photos or videos on this computer which I haven't got on any backup drive before I format and re-install