Screensaver started and locked the "guest account" and asks for password.

I were logged in as a guest user on Ubuntu 12.04. I left the computer for a while and when I came back the screensaver had started, and locked the computer. Now it asks for a password to log in to the guest account. I don't have to write a password when I logged in as guest. How do I enter my current guest session without restarting the computer?