Should be in appdata local low I believe.

Were you playing on a server or solo? Server worlds are kept on server side.

Or do you mean you have a world and it won't open? My understanding is it may take some time to update it. I have a lot of old worlds but I haven't tried to go back to any of them, so I don't really know how long it may take. People have said it goes black screen for a while before loading.