This scenario usually occurs when you upgrade moodle. If upgrading from moodle-1.9 to moodle-2.1.1, there is a database engine compatibility issue.
In the moodle notification page, the message below usually pops up:
Database tables are using MyISAM database engine, it is recommended to use ACID compliant engine with full transaction support such as InnoDB.
To convert your database engine to InnoDB, run the innodb.php script in the directory of the admin directory which is resident in the moodle directory.
http://moodlesite.domain/admin/innodb.php
Replace moodlesite.domain with you moodle path or IP address.
In the moodle notification page, the message below usually pops up:
Database tables are using MyISAM database engine, it is recommended to use ACID compliant engine with full transaction support such as InnoDB.
To convert your database engine to InnoDB, run the innodb.php script in the directory of the admin directory which is resident in the moodle directory.
http://moodlesite.domain/admin/innodb.php
Replace moodlesite.domain with you moodle path or IP address.