Drupal/WissKI is a complex framework where errors can be provoked by wrong configurations or incomplete routines. Even with careful testing, errors sometimes creep into the code. In both cases you are confronted with Drupal's notorious whitescreen "The website encountered an unexpected error. Please try again later.", which provides little information at first.
You have two options to get more information:
1. Navigate to Manage Reports Recent log messages, where you can see what happened.
2. Add the code line
$config['system.logging']['error_level'] = 'verbose'; to
The last option is especially useful when the entire system is broken and you can't get to the administration view anymore.
Some common Errors with simple solutions are listed here, for others please visit the issue section on drupal.org.