I booted up minecraft this morning and went to play singleplayer. my save file wasn't there. i was worried at first but then i checked .minecraft/saves/ and it was there. I backed it up now. minecraft still cant find it though when i try to play it. i even tried re-installing the whole thing and it still didn't work. please help
It's possible that your level.dat for that world might be corrupt. Create a new world with the settings of your old one (e.g. survival, no cheats) and then copy its level.dat into your old world. The file will be in its world folder in .minecraft\saves. Seeing as you have a backup, you can restore it should anything go wrong.
Stu