How Do You Troubleshoot RDP Not Working?

Problem scenario
RDP is not allowing you to log into a Windows Server. What should you do?

Possible Solutions (that can be followed in random order)

1. Is port 3389 blocked? Are you using iptables or a public cloud? Check egress ports from the client and ingress ports on the Windows server. You can use PowerShell or nmap to test port 3389.
2. Are Remote Desktop Services running on the server?
3. Can you make sure the Domain Controller is working correctly with the Windows server?
4. Do you get a vague "internal error" when it fails. You may have misconfigured Network Level Authentication (NLA). Can you temporarily disable NLA on the Windows server? See this Microsoft article for further information.
5. Can you use PowerShell or some other method to log in? Maybe your credentials are not working at all.
6. Does the RDP client work with other Windows machines? If not, can you use a new RDP client? There are different clients for Windows and Linux.
7. See this Microsoft article on troubleshooting RDP.