Well shit. I don't know if this is happening to any of you guys, but I've been having some major issues with playing the user-generated maps in Portal 2. The same problems occur even if I start up the single player mode. These problems are also existent in the original Portal.
You see, whenever I select a mode, generally, it takes me to the loading screen. Once it loads all the way, the game simply crashes. These problems didn't occur when I first played the game last weekend. I don't know whether it's a bug in the system or my graphics card or anything like that. Though I do know one thing; I'm not gonna be playing Portal for awhile.