I haven't seen this exact situation so I thought I'd make a post instead of messing with some fixes that might do more harm than good. I have already tried using the level.dat_old file and unfortunately it didnt work.

So I have been making good progress on my mines and hillside fortress. I open cartograph and then save and quit minecraft. I went back to the select world screen to see how large my world was. It was at 8.75, but when I look now its at 3.74. The first time I opened the world after I noticed something was wrong, I seemed to be underground and constantly took damage until I died. I respawned, ran to my base, and saw there was nothing there. My world has been totally reset. Same world, just wiped clean of everything i built. I'm assuming its 3.74 because it has still included my limited adventuring. All of my progress, however, is gone.

Minecraft did not crash. It just seemed to revert to the original world without any of my progress. I don't have a backup and level.dat_old has the same problem. I've searched the forums and didnt find anything specific to my problem. I was making really nice progress and would like to have my world back... help?

Would this work? viewtopic.php?f=17&t=51286&hilit=fix+corrupt+save
If so, I don't understand how to make the level.dat template. Help would be appreciated.