jumanji said:
If the BIOS does not see the disk, then we can't do anything. No data recovery software can see it too. Live Linux also will not see it. You have to seek professional data recovery agencies for data recovery.

The other 500GB disk which also you said had turned "unknown and not initialised" seems to be fine.

And so that is it.
yep, that's what I thought too. Fortunately, that disk was the backups disk, so now I just need to backup everything to the new 1TB disk again and we should be fine.

Thanks anyway for all your help!