Steps:

1- Check if you receive the same error in www.yourwebsite/wp-admin. If this is the case, proceed to repair the database. Add the nex code in wp-config, at the end:

define('WP_ALLOW_REPAIR', true);

After that, enter in your website’s url:  http://www.yoursite.com/wp-admin/maint/repair.php

And repair your database.

2- If your receive different messages in fron and back part, you should check the datanase connection, in wp-config too:

define(‘DB_NAME’, ‘database-name’);

define(‘DB_USER’, ‘database-username’);

define(‘DB_PASSWORD’, ‘database-password’);

define(‘DB_HOST’, ‘localhost’);