Hi Rene,

You need to login to the built-in admin account and check:
1.Click on Start icon and select Command Prompt run as administrator there.
2.Accept the UAC prompt.
3. Enter the following command to enable the hidden administrator account: net user administrator /active:yes
Note: To disable the account follow the instructions : net user administrator /active:no
If you can log into the Default Administrator account, follow these steps to perform System Restore:
a.On the desktop, press Windows key + R to open a Run Dialog.
b.Type %systemroot%\system32\rstrui.exe.
c.Press Enter and select a restore point before the account got deleted.
Check if the account is restored.

I hope this helps.

Regards,
Lokesh