There are a number of steps you can take just within Drupal 8 that will help you diagnose problems in your site. If you have Drupal Console installed, you can enable many of these modules by running drupal site:mode dev. Many of the debugging settings are controlled in the /sites/default/services.yml file.
If there isn't one already, you can copy the starting /sites/default/default.services.yml file to the services.yml file to start with.
