Singleplayer doesn't use player.dat files, only level.dat (which contains a copy of the data that would be in your player.dat in multiplayer).If the world is still in your save folder, but you can't see it in game, there's a fair chance it's not the world, but the player.dat file that's corrupted. Try making a new world, exiting, then coping the player.xxx files from that world to the old one. If you can see the world in game, you're golden, though you may need to cheat in anything you had in your inventory.