When Joomla breaks, it can do so in many ways, mostly giving a clue as the issue. In some cases however you just get a white screen with optionally the word "Error". I could list out the many things this could be, but its much easier to just turn on debug and see.

You'll need to connect to your website using whatever tool you use, probably FTP (If you're using a GEN host then you'll find the file manager in the admin console).

Locate the file /configuration.php - this file contains the initial config for Joomla. Edit this file and locate the lines…

        public $debug = '0';
        public $error_reporting = 'default';

Now change this to:

        public $debug = '1';
        public $error_reporting = 'maximum';

and upload the changes to your site. Now refresh the page and you should see some useful diagnostics.

When you've solved the issue, return this back to its previous settings.

If you have a joomla website and its down, and you would like professional help, GEN provides support and maintenance for Joomla, https://www.gen.net.uk/services/hosting/3788-joomla-cms-development