Probably the map corrupted, that same thing happened to me I even made a thread about it
http://www.frictionalgames.com/forum/thread-24397.html and if the weight of the map still shows and the hps file and everything still works and the map just comes blank in the level editor just like what happened to me, then that means it's corrupted! you can't use that map anymore, it has been deleted, so unfortunately you have to make a new one now
You should always keep a backup file of your maps, that's what I do