You broke you site didn't you? and now the error: "User '' has exceeded the 'max_questions' has you locked out of the from end and back end.
Don't worry, all you have to do is wait a while (probably)
I got the error: User '<random number>' has exceeded the 'max_questions' resource (current value: 75000) SQL=INSERT INTO `jml_session` (`session_id`, `client_id`, `time`) VALUES etc.
The first thing I did was contact my host. I recommend you do this just in case. It turns out the error was cause by a plugin which simply exceeded the 'max_questions' resource. My host didn't actually help but after about 20 mins (my host company suggested it could take up to two hours), the 'max_questions' resource was reset and I have access to both the front and backend. If you don't want to bother contacting you hosting provider, just wait 20 mins an occasionally refresh.
If the problem persists you host might be able to raise the resource but they may charge you for that.
In any case when this happens there is no indication that waiting is the only thing you need to do to get rid of the Joomla error:
"User '<random number>' has exceeded the 'max_questions' resource (current value: 75000) SQL=INSERT INTO `jml_session` (`session_id`, `client_id`, `time`) VALUES etc"
If this error appears for Wordpress, Drupal or any other CMS, I'm sure the soltuion will be the same. Wait.
Hope you are back online soon!