If you find that you can no longer close the screensaver in Windows 10 by moving your mouse or pressing a key on the keyboard, we have found the most common cause of this is a setting in the screensaver.
To fix this, follow these steps:
Search for Screen saver in the Windows 10 search bar and select Change screen saver. Otherwise you can go Settings > Personalisation > Lock Screen and scroll down to the very bottom and click on Screen saver settings
Under the Screen saver tab change “Windows Gallery” to Photos
This should resolve the issue for you.