Need help getting website back up – theme and a plugin causing the website to present

This is a website that I haven't looked at for ages and didn't realise has been down since August! Whoops! I can't access the site or the wp-admin and the ftp isn't working either. I've worked out that I need to turn on a different theme and disable a plug-in in order to get to the wp-admin but when I go to phpmyadmin, I get lost and am instantly out of my depth! This is the site error message:

Strict Standards: Non-static method IS_IU_Import_Users::init() should not be called statically in /home/content/p3pnexwpnas05_data02/30/2382330/html/wp-content/plugins/import-users-from-csv/import-users-from-csv.php on line 374

Fatal error: Call to undefined method Fusion_Helper::fusion_auto_calculate_accent_color() in /home/content/p3pnexwpnas05_data02/30/2382330/html/wp-content/themes/Avada/includes/upgrade/class-avada-upgrade-552.php on line 143