How Do You Get a DNS Server That Was Working to Work Again?

Problem scenario
Your DNS server does not seem to be working any more.  What should you do?

Solution
1.  Can you ping it? 
2.  Log into it.  Run these commands to ensure that there is activity on a port:

sudo netstat -anlp | grep :53
nmap -Pn 127.0.0.1

If there is no activity, start the service with this command:
sudo systemctl start named.service