I was playing on my world I had been playing on for months and I saved and exited and when I returned later, the world was not there. Has anyone had this happen to them? If so, were you able to get your world back?
Is the directory for that world still in your saves directory?
If so, and the data files are still in the region directory I think there was a post (probably by TheMasterCaver) that explained how to get Minecraft to recognize it as a valid world again.
(I don't remember anything about how though.)
This is done by copying "level.dat_old" and renaming it to "level.dat", replacing the one that already exists (if it does, as I've had it get deleted after a crash; rarely, even level.dat_old gets deleted, in which case you'll have to create a new world, ideally with the same seed and world settings, and copy its level.dat over, which will cause you to lose your inventory (it may be possible to delete the player data, under the "Player" tag, with NBTExplorer so the game reads it from the player data file instead, or copy the "Player tag" in player.dat file to level.dat). There may be other issues as well if you completely replace level.dat since the game stores information like the status of the dragon fight and locations of end gateways (I don't know what happens if this data is reset or changed).
Also, if a world is old enough, from around 1.6.4 or earlier, it will also have a "level.dat_mcr" file even if it wasn't converted from MCRegion (pre-1.2), which can be used to recover the seed in the case level.dat and level.dat_old are corrupted (unlike level.dat_old it will not contain much of anything else unless the world was actually converted).
Note: the Wiki still erroneously claims that "level.dat_old" is a backup from converting an Alpha world to a newer version, which is not true (at least, not for a very long time); I mention this because it may make people trying to fix their world think it is some old file.