If you can't access your site in a web browser, please contact your website hosting provider.
Your hosting provider can investigate why your site isn't working, and help you get it back up and running. The WordPress app can only connect to your site when WordPress is installed and working there, so if you can't access your site at all, the app (and the app support team) can't fix that.
You can also check the Common WordPress Errors support page for common problems and how to resolve them on your site.
Once your site is working again, you can try to connect to it and manage it within the app.