Random Occurrences of Blank Contribution Thank You Pages
This issue is related to Drupal 7 (possibly wordpress), PHP prior to 7.3 and Chrome 80+
To fix, upgrade to PHP 7.3 and add the following text somewhere in your drupal 7 settings.php file.
ini_set('session.cookie_secure', 1);
ini_set('session.cookie_samesite', 'None');

or you can style it like we did so you remember why you added this.
/**
 * Fix for Chrome Cookies Issue - See:
 * https://www.zoocha.com/news/how-fix-chrome-80-cookies-issue-drupal &amp; 
 * https://lab.civicrm.org/dev/core/-/issues/2059
*/
ini_set('session.cookie_secure', 1);
ini_set('session.cookie_samesite', 'None');

Clear CiviCRM and Drupal caches.
Clear browser cookies and cache.
(Unfortunately for your returning users, unless you have them clear their browser cache/cookies, they might experience the blank screen again.)
If you do not want to upgrade to PHP7.3 the zoocha article explains how to fix in previous versions.