Possible problems are -

Password not correct.
Cookies not enabled (they need to be on).
You have arrived at this page without logging on.
There has been 30 minutes of inactivity which causes you to be automatically logged out.
You are very unlucky and this web site has just been updated and re-started your session!

95% of the time this can be resolved by simply logging in again; if this fails please e-mail me at koala@koalacomics.com.au