Site in offline mode, any way to debug how this happened?
Site offline is a matter of a global variable site_offline set to 1. Any call of variable_set('site_offline', '1') can also result to put site into offline mode.

Invalid database configuration can also results as site offline but it adds some error message.

Unauthorized access to DB can be used to manually alter record in variable table for the values as
variable.name='site_offline', variable.value='s:1:"1"'