The cause of this error is MySQL database file corruption. It can occur after the server is shut down suddenly – especially after power failure or if the disk is physically damaged. Luckily, this problem can be easily fixed in most cases.
You will need to run a MySQL database fixing operations to repair the tables. This is usually done by using the features in your hosting control panel.
For example, in Cpanel based accounts, go to 'MySQL Databases' page and click on the 'Repair' link next to the database name.
This can also be done from PHPMyAdmin, where you can select all the database tables in the database, and select 'Repair Table'.
(Note: The error can occur with other tables, not just config.MYI – depending on which table was corrupted)