UPDATE:
So, I managed to get the HTML working again by using the following commands:
chown -R www-data:www-data /var/www/html
chmod -R 775 /var/www/html
BUT, now the images do not show up.
After looking in the Google Chrome “Developer tools”, I can see a bunch of “404 error” messages in the console. Here’s what the error messages look like:
It seems to be some error with the redirect. Even the HTML “email preview” page is completely blank after clicking on the “preview” button. HTML not blank in the 'Builder" (editor),but the whole template HTML & images are completely blank.