I fixed this by going through PHPMyAdmin to the table “Options” and then the row active_plugins.
I had the following stored there (formatted for readability):
a:10:{
i:0;s:49:"1and1-wordpress-wizard/1and1-wordpress-wizard.php";
i:1;s:29:"acf-repeater/acf-repeater.php";
i:2;s:30:"advanced-custom-fields/acf.php";
i:3;s:45:"limit-login-attempts/limit-login-attempts.php";
i:4;s:27:"redirection/redirection.php";
i:6;s:33:"w3-total-cache/w3-total-cache.php";
i:7;s:41:"wordpress-importer/wordpress-importer.php";
i:8;s:24:"wordpress-seo/wp-seo.php";
i:9;s:34:"wpml-string-translation/plugin.php";
i:10;s:38:"wpml-translation-management/plugin.php";
}
I added a new line (for the missing plugin) and incremented the a:10 to a:11 to indicate that there are now 11 items in the array:
a:11:{
i:0;s:49:"1and1-wordpress-wizard/1and1-wordpress-wizard.php";
i:1;s:29:"acf-repeater/acf-repeater.php";
i:2;s:30:"advanced-custom-fields/acf.php";
i:3;s:45:"limit-login-attempts/limit-login-attempts.php";
i:4;s:27:"redirection/redirection.php";
i:5;s:40:"sitepress-multilingual-cms/sitepress.php";
i:6;s:33:"w3-total-cache/w3-total-cache.php";
i:7;s:41:"wordpress-importer/wordpress-importer.php";
i:8;s:24:"wordpress-seo/wp-seo.php";
i:9;s:34:"wpml-string-translation/plugin.php";
i:10;s:38:"wpml-translation-management/plugin.php";
}
i: appears to be item number, and thanks to JHoffmann’s comment, it appears s: is the length of the string that follows.
The site now works as before!