Clean URLs caused site to break?
As leymannx and Kevin mention it is probably a misconfigured tmp/files directory

Uncheck aggregate css is something that you would not want on your live site as the performance will suffer.

Check in Reports > status report (/admin/reports/status) and try to fix or report on the errors that you see.  

Also go into configuration > media > file system (/admin/config/media/file-system) and correct any file system problems reported when saving the settings.

The most probable cause of what you are seeing is wrong permissions.

There is a lot of guides on the net about setting those permissions. 
You can start from the official guide on the subject or on one support thread that started as a drupal 7 thread but has Drupal 8 answers.