So i go to change the skybox model on a map that i had just about completed and now Radiant won't display the 3d view of the map or let me edit it, and uses a huge amount CPU when I try to load the map. Normally this isn't an issue because you can change the KVP's and restart and everything will be good, i can't do that this time.

Does anyone know how to get it unstuck?

Edit the map file in notepad (make a backup first!), find the skyboxmodel key and delete its value.