(Now I'm sure it was a technical issue)
When I load a new floor in a mission or the Arcade, my knight locks up in the loading posture, like when you're the first one to load into Lockdown and everyone else is standing upright.

All of the menus are available; I can get out of the mission without logging but my knight won't load in properly. No movement, no input, just frozen in place.

I've never had this happen before but I stopped playing for about a year and when I came back it recently it started happening.

EDIT: After reading several guides calling for changes to the java_vm directory in SK's install folder, I saw I did not have one. I created one and copied the contents of Java's jre folder into it.

http://forums.spiralknights.com/en/node/62691

That's the guide I used.