HTTP
401 Unauthorized
What it means
The server understood the request but will not fulfil it without valid authentication. You reached the right machine. It is asking you to prove who you are, or the proof you sent was missing or wrong.
Where you usually see it
Login pages, APIs, and admin areas of a website. Some applications also show it after a session expires.
What to try first
If you have an account, sign in again and retry the same address. If you were already signed in, sign out, sign in, and avoid using a stale bookmark that points at an old session. Check that the address is the one you intended, not a look-alike host.
When to stop
A 401 is not a broken internet connection and not a reason to change DNS. If you run the site, look at whether the resource actually requires a login and whether the credentials or token you are sending match what the server expects. Do not post your password into a troubleshooting forum.