I have been playing on this survival world for about 2 weeks, and i have built alot of things. Today, randomly while i was playing I got the blue screen. No idea why. But anyways, when i restarted and opened up minecraft and clicked on single player, my world did not show up. I tried to problem solve, and i opened up the .minecraft folder and went into saves. My single player was still there. I changed the name of it, hoping it would fix it. It still does not show up.
All the world files are still in my world save. It's just not showing up?
Usually this is the result of a corrupted level.dat, as mentioned above sometimes mcedit will regenerate level.dat if you open the world and save it within mcedit (note minecraft itself must be closed during this process). If that doesn't fix it then I'd suggest you look at the world recovery thread.
I just had this same problem. I opened it in MCEdit, then saved it (ctrl-s). It then showed up in my list, but loading it, my spawn point appeared to now be in both solid rock and lava, as I couldn't move and burst into flames (salt in the wound, minecraft!) So I went back to MCEdit and manually set the spawn point somewhere safe, and now it appears to be working fine again. So if you spawn somewhere weird (or fatal) after the first step, the world might not be totally lost; just move the spawn point.