Recently I upgraded a from 4.7 to 5.1. All was well and then after a weekend suddenly the CiviCRM menu has disappeared and the dashboard and most other pages are displaying the error bubble:

Network Error Unable to reach the server. Please refresh this page in your browser and try again.

Thinking it was some dastardly upgrade gremlin i decided to upgrade to 5.2 which has made no effect. I've checked the base url - with and without HTTPS. Any other troubleshooting tips I can try to resolve this?