I had the same problem after installing CKEditor.
Usually I install modules via Backend, but this time I uploaded the module and used »install.php« from modules/advanced. After that my backend was gone! And then, there was a much bigger shock as i found that my frontend was gone as well.
I found that some database entries in settings table where deleted:
- default_template
- page_extension
- page_spacer
- pages_directory
- page_extension
- media_directory
- wbmailer_routine
- server_email
- default_theme
... and some more I think.
After inserting the default_template, my backend was back an I could insert the other missing entries.