Have a power outage or something and lose the world you where working on? Same thing happened to me when the power suddenly went out at my home. If you're level still exists, one of the world files (or maybe even both of them) are corrupted.
In order to fix this, pull up minecraft, go to the button that's dedicated to texture packs and mods, and click the button that says "Open texture pack folder". From the folder that pops up, you should be able to trace it back to just the minecraft folder. If you're running Windows 7, just click the word minecraft in the file path, and it'll take you straight there.
After getting to the minecraft folder, go to your "saves" folder. In that folder should be a list of your worlds. Click on the one you're having trouble with. If it's not there, I can't help you. If it is, read on.
Since the file has been corrupted, it needs to be slightly changed in order to work. There should be two files in the world folder you selected. One is called level.dat and the other is level.dat_old.
If the following steps do not work, don't worry, we still have one more option.
1. Rename the level.dat file to level.dat_broken
2. Rename the level.dat_old file to level.dat
If that didn't help, there's only one option left. Find a seed that you like and generate a new world. If you don't like the world you generated, redo it until you do, because this new seed is what you'll be stuck with. After you've found a world you like, go back to the save folder from earlier (the one you got to from the mods and texture packs folder). Select the new world from the save files list, and open it up. There, you should see the level.dat and the level.dat old just like the other one. Here's the trick that will either make or break you, so listen well:
Take that level.dat file and move it into the old world's folder. If a window pops up and tells you that it already exists and asked if the old level.dat should be overwritten select Yes. Now open up you're minecraft. Open up the world you lost, and it should look exactly like the world you JUST GENERATED. Don't panic! Just use cartograph on that world, and you should see you're building and such!