Last tested: Jan 21, 2019
For Looker-hosted instances: Your web browser talks to a load balancer which forwards the traffic onto nginx reverse proxy servers. Nginx then forwards the traffic to the appropriate looker. That error message means that nginx didn't get a response from Looker in a timely manner.
In short, Looker was/is down. Logs should help us understand the root cause - if your Looker deployment is hosted by Looker, please open a Looker support request for assistance.