Click/Tap on the Power button under the Start Menu or on the lower right-hand side of the Login screen, press and hold the Shift key, and click/tap on Restart. This will open a command prompt at boot.
net user Administrator /active:yes
If for some reason that doesn't work, do the following:
In the command prompt, type regedit, and press Enter.
In the left pane of Registry Editor, click/tap on the HKEYLOCALMACHINE key.
Click/tap on File (menu bar), and on Load Hive.
Open the drive (e.g. C:\ ) that you have Windows 10 installed on, and browse to the location below. NOTE: The drive letter (ex: C) will not always be the same at boot as it is from within Windows 10.
Select the SAM file, and click/tap on Open.
In the Load Hive dialog, type REM_SAM, and click/tap on OK.
In the left pane of Registry Editor, navigate to and open the key: HKEYLOCALMACHINE\REM_SAM\SAM\Domains\Accounts\Users\000001F4
In the right pane of the 000001F4 key, double click/tap on the F binary value to modify it.
In line 0038, change 11 to 10, click/tap on OK.
Close Registry Editor and the command prompt.
Click/tap on Continue to startup back in Windows 10.
NOTE: To disable the Administrator account simply reverse 10. to 11