The Your computer is locked screen locker is a Trojan that displays a fake security screen stating that the computer has been locked because viruses were detected. It then tells you to contact a Microsoft technician to get a an unlock code to unlock it. Once you enter the correct code, the screen will unlock and another screen will be displayed that contains instructions on how to remove the Trojan. If the unlock codes or the Alt+F4 keyboard combination do not properly shut down this screenlocker, you can use the removal guide below to remove it for free.
Source: https://www.bleepingcomputer.com/virus-removal/remove-the-your-computer-is-locked-screenlocker