Execution of a query to the database failed - Incorrect key file for table ’./kathleen_modx/modx_site_content.MYI’; try to repair it »
This is MYSQL telling you that the table has been damaged/corrupted somehow and it needs to be repaired. It’s not a MODx problem per se.
You can do this through tools such as phpMyAdmin or the MYSQL CLI client.
If you are unsure here ask your service provider first, show them the error, they may do this for you. This is not a difficult thing to do, however please make sure you have a database backup before you do this, just in case.
Use MODx, or the cat gets it!