Everything is black.

I'm super new to mapping and I have only completed one map which is just a room with a skybox on the top. I am making a small tf2 koth map and I have so far completed the spawn room. I wanted to compile the map but I didn't want to have to enclose the map with a roof. So I made a giant hollow cube of skybox around the map thinking this would stop leaks. When I compiled it everything was just black. Why is it not working and how do I fix it?