Your level.dat in the save folder might had gone missing.... Dont believe that is caused by the update.
If that is the case and you still want to recover your world, while loosing your inventory just create a new world with a different name and then copy the level.dat file from the new world to the old world. MC will see the world again but... you will lose all your inventory and quest/thaumcraft(..) progress, since it is saved inside this file.
You can always go nuts, and edit the level.dat file with an NBT Editor and copy the contents of the player.dat file inside player folder to the level.dat file.
In single player, the player/player.dat file is a copy of your player info that is inside the level.dat but its not used. Its just there for server compatibility reasons. Single player cliente uses the player data inside level.dat.
If that is the case and you still want to recover your world, while loosing your inventory just create a new world with a different name and then copy the level.dat file from the new world to the old world. MC will see the world again but... you will lose all your inventory and quest/thaumcraft(..) progress, since it is saved inside this file.
You can always go nuts, and edit the level.dat file with an NBT Editor and copy the contents of the player.dat file inside player folder to the level.dat file.
In single player, the player/player.dat file is a copy of your player info that is inside the level.dat but its not used. Its just there for server compatibility reasons. Single player cliente uses the player data inside level.dat.