Why Nginx Ubuntu 14.04.5 x64 stop working suddenly?

I followed this tutorial to set up a Nginx.

Criteria like below:

• Image : Ubuntu 14.04.5 x64
• Region : SGP1

It works for the first time.What I mean it works is :

• Able to access from web browser. (I print Hello world in a HTML file)
• Able to access from putty with the credential I set earlier.
• Able to access the file via FileZilla.
• Able to call to API via Postman,getting the response as expected.

Then suddenly it stop working.What I mean stop working is:

• Cant access the html that working earlier via browser.
• Access with putty get ->Network error: Connection Timed out
• Unable to call to API via Postman,getting this response : Could not get any response
• Cant access with FileZilla get this error below

Error: Connection timed out after 20 seconds of inactivity Error: Could not connect to server

So in order for me to make it connect again,I tried the following:

1. Disable the firewall using: sudo ufw disable
2. Restart nginx using : sudo service nginx restart
3. Restart the droplet in digital ocean console
4. check the nginx status using : sudo nginx -t and get the nginx is working fine

Now when I access the droplet via digital ocean console,I get this result on screen:

Result on screen when access from digital ocean console

What I missing out??This situation happened 2 times ady…First time I delete the droplet and start over.But now it happened again.

Question: So tell me what to do to avoid this happened again.And how to solve this problem to make the server work again?