Contact your hosting provider to get more information about that. They should be able to guide you and tell you what happens. I suppose that you reach some server limit and it caused the problem.
If you are getting the 500 error in Customizer – Contact your hosting support to get help with the increase of SubstituteMaxLineLength. As a rule, this limit caused the 500 error of the Customizer. To be 100% sure provide us with the server error logs. There should be the error that explains what limit is reached.