You're missing a couple of things in your post that will help us help you:

- Your Zen Cart version
- The PHP version on which your site runs. Since you can't see the admin, you can most likely see that version setting in your webhost's cPanel's "Multiple PHP Version Manager".
- Have you checked your site's email settings? See https://mxtoolbox.com/
- Are there any PHP logs in the site's /logs directory? You can view that directory via an FTP-access or via the cPanel "File Manager".