Custom certificate (plugin)

Hi, I am receiving the same error in our moodle. My moodle version is 2.5.2 We are using moodle from last 7 years we didnot face this type of problem at any time but last one months we were faced so many problems like 1. quiz conducting time automatically question bank got deleted 2. quiz grades are getting none status without doing anything 3. Enrollment option got deleted automatically. Please help me on this. The following message i have received Cannot start an attempt at this quiz. The quiz definition includes a question that does not exist. More information about this error Please help me to conduct the quizzes. Thanks & regards,