please use your win10 bootable disk to boot your computer, then choose repair, in command line(make sure your windows system is located on c drive or you have to change the driver letter from c to the system drive letter):
move c:\windows\system32\utilman.exe c:\windows\system32\utilman.exe.bak
copy c:\windows\system32\cmd.exe c:\windows\system32\utilman.exe
then you can reboot your computer,
in the windows login screen, on the right hand corner, you can find the 2nd icon: utility manager, click it and click on on screen keyboard,
it will open the command prompt, then you type command below：
net user /add john *
net localgroup administrators /add john
you might need restart you computer again to see the new user called john, and login with the password you give.
Best of luck to you!