error establishing a db connection !


I have a site that recently more than 90% when trying to access the site through Desktop or Mobile the user will see an error message ( can't establish a DB connection ) !
the same issue when admin users try accessing the admin panel !

I tried to analyze it but I can't see any issue , I tried cleaning the DB using
define('WP_ALLOW_REPAIR', true);

but it didn't get nay issues and says that everything is clean !

I was hoping if you can help me on this as it making me go nuts