I'm having the same problem. I got sorrounded by zombies earlier and had no bandages left, so when my character fainted I knew it was over and tried to respawn. I got frustrated when I realized that you can't hit escape when you're unconscious, not wanting to wait until I bled out, I hit Alt+F4. I'm now getting stuck on the loading screen no matter what server I try to join, I'm assuming it has something to do with the Alt+F4 process, it broke our characters.