To improve your experience with Dynamics 365, your organization has been moved to another set of servers. When this move was made, the cross-reference that points your organization’s Web address to the new server was updated. For some subscribers, their Internet Service Providers update cross-references only one time every 24 hours. Until then, you may be unable to sign in to your organization. For more information about Domain Name System (DNS) service entries, read this Knowledge Base article: article
What [most likely] has happened here is that you changed the version of the organization and, as a result, it’s been moved to a different scaling group with a different IP address. Unless it’s intentional, try to avoid changing the org version – that will minimize chances of your cheese being moved.
The KB article suggests waiting it out but sometimes these simple steps can get you out of trouble: close the browser, open command prompt, run ipconfig /flushdns command, start the browser.
Thanks to David “Xrm.Tools” Yack for the tip!