Error establishing a database connection

[JxstErg1]

Please read this:

Wordpress and website can't connect to SQL Database

Yeah it seems to be caused by your website opening unusual amount of database connections, which can be caused by either so many traffics (though it should not trigger it unless in rare circumcations) or you have installed plugins which open many database connections in background.

and also the plenty of topics with answers while searching for max_user_connections, as that error has been discussed so many times now.