Log out to the sign in screen.
Hold down the shift key on your keyboard while clicking the Power button on the screen.
Continue to hold down the shift key while clicking Restart.
Continue to hold down the shift key until the Advanced Recovery Options menu appears.
Click Troubleshoot
Click Advanced options
Click Command Prompt
At the command prompt, type:
net user administrator /active:yes
Hit Enter
Close command prompt, restart, then try signing into the Administrator account.
Try signing in again.
Reverse the Group Policy Restrictions you applied.