I was messing with the configuration of my local Bugzilla, and I changed the Bugzilla offline message from nothing to some text.
Now, it has logged me off and I can't go back in to change it, or find where it's storing the text in the database.
Unfortunately none of the solutions offered here or on the Bugzilla / Bugzilla tracking system were able to solve the problem.
The installation is IIS7.5 / Window Server 2008 R2 / ActivePerl.
Settings were changed manually in the /data/params file.