Monitor is saying "input signal not supported".What have I done wrong?

Originally I have just used my smaller monitor for my computer. But this recent Black Friday I got this new 43" 4K UHD smart tv. And I wanted to use it as a smart tv (of course) and also as another display for my computer, so I'd have a dual display.

But somehow wound up having my original screen saying "Input signal not supported" and my flatscreen just giving me no signal.

What have I done wrong? And how do I get it back to normal?