Sooner or later, it's going to happen. You'll apply a module update or do some other under-the-hood task and the next thing you know, your Drupal 8 site is no longer responding (often called whitescreening). While whitescreening is a good security practice (as opposed to the site spewing error messages to the browser that might reveal sensitive details), it's a royal pain when you just want your site to work and you have no idea what's wrong or where to start with fixing it.
What follows are some tips and advice that might get you back in action again without (completely) losing your sanity.
Backup, Backup, Backup (and Restore)
I'll go ahead and get this one out of the way, even though it may be too late at this point to be of any help. In the future, always have an automatic backup generation plan in place. The typical advice is to always make a backup before doing anything dangerous to your site, like applying patches and updates, but it's often when you don't think your action could be very dangerous that it ends up frying the whole site.