If you encounter a secure connection issue on your website, there are a few possible reasons. Here are the most common ones and how to fix them:
If you see the ERR_SSL_PROTOCOL_ERROR, it means that an SSL certificate is either not installed or not working properly on your website. If the certificate is already installed, check this guide: How to Fix ERR_SSL_PROTOCOL_ERROR
Add https:// to your URL in the browser's address bar. If it loads correctly (there's a padlock next to the web address), then you only need to force HTTPS so that the visitors would land on the secured version of your website by default
If your website is accessible through HTTPS, but its layout is distorted, it's likely due to mixed content issues. Refer here to fix it: What to Do if a Website Shows a Mixed Content Warning
You can also check this video tutorial: