I have a MySQL error on my website
The most common reason for this problem is that the database credentials in your website’s application configuration file are invalid. Verify them and check if your MySQL user has been added to access the database. You can manage your MySQL databases, users and their password from Site Tools -> MySQL Manager. WordPress users can follow the instructions in this article on how to resolve the problem.
Another possible cause could be that the MySQL query you are trying to execute takes too long and the MySQL server times out. The solution for this is to optimize your database for the queries which fail.
You can also have a crashed table in your database as the cause of the error. The solution is to repair and optimize the entire database.
If the problem persists, restore your database from a backup created when the site was working properly.