If you have enabled Windows 10's built-in administrator account, and upgrade from build 1803 to 1809 -- that is, the April 2018 Update to the October 2018 update -- you will find that a bug has been introduced that disables the account.
Microsoft says that it is aware of the problem, but the company does not plan on releasing a fix for it until the end of the month.
The issue has been noticed by users -- as well as numerous tech blogs including MSPoweruser, and has also been highlighted by Microsoft's Japanese team in a post on TechNet. The post points out that in order for the problem to appear, two conditions need to be met. Firstly, the built-in administrator account must be activated, and secondly that there must be another account with administrator privileges.
The post goes onto say (Japanese-to-English translation via Google Translate):
We are currently working on solving this problem, and at the moment we aim to release a patch in late January 2019.
Therefore, if you need to upgrade before the patch is released, please confirm that you can surely sign in with a user with administrative privileges other than the built-in administrator, before you upgrade Please do it.
Also, if you have already upgraded and you have invalidated the built-in Administrator, you are signed in as a user with administrator privileges other than the built-in Administrator, activate the built-in Administrator.