I'm playing Minecraft v1.7.5 on Vista (i know!).
Mine and my son's creative mode game disappeared from the menu - i think it might have been accidentally deleted. The folder was still there in
<user>\AppData\Roaming\.minecraft\saves\Arlo's World, but there was no
level.dat_old in it.
I used Recuva to search for deleted level.dat files and it found a
level.dat_old file from that folder that could be recovered. I recovered that, put it back in the folder, copied it and renamed the copy to
level.dat, so that the
.minecraft\saves\Arlo's World folder had a
level.dat and a
level.dat_old file again.
But, on starting the game, it still doesn't show up in the list of worlds in the single player game list.
Is there something else I need to do, to make the game add it to the list of options? Comparing the world folder with a still-working world, it looks like everything else is still present.