How to find out why my server not responding?

How can I find out why my server not responding?
curl http://mydomain.com
Response: (56) Recv failure: Operation timed out"
After server reboot:
mup logs --tail=1000
shows "Starting meteor app on port:80"
But page loading-loading-loading…


Digital Ocean, Ubuntu 16.04, Meteor 1.5.1, Mup 1.3.1