We're running a site that's had PHP upgraded on our server (hosting provider) which I'm finding I think is incompatible with a few of my modules. Initially the site went into maintenance mode. Changing the settings.php "$db_url" line from = 'mysqli: to 'mysql: (without the 'i') fixed this issue and brought the site backup. Host said something about it not being supported in the version of PHP we'd been upgraded to?