Autoswitch monitors on PC?

I have a dual monitor setup: desktop monitor and TV. I use Win+P to set up the TV as screen only. Now, few months back, it happened that when I tued the TV off in this setting, the monitor would tu itself on, and back off if the TV went on. Great setup for me. However, some update apparently killed said setup. How do I get it back again? Thanks!

The monitor is on DVI, the TV on HDMI output.