So you haven't uploaded any new Tiki files or tried to upgrade recently?
To answer your debug question: Yes, I can't remember where, I think under General Admin. You might also have to turn on the debug panel under "look and feel" too.
Update: I just had a look myself and to turn the debugging messages on it is under General administration option. Plus, as stated you will need to turn on the "debugger console" under "Look and Feel" administration option. It will then appear at the bottom of the page (at least it did for me).