I have done a little research, and most of the time this issue was reported, it was solved by restarting Steam, Java, or their computer. If this is the case for you, I can assume that you have already fixed your issue. Otherwise, it would be helpful to post basic information about your computer as well as which Spiral Knights client you use, so that if your problem has not been solved already, someone more knowledgeable would be able to step in and help.