It's a phenomenon that's been observed in the most advanced graphics rendering engines. A pixel goes rogue, decides it's had enough of being squared off, and just... breaks. Like this:
As a result, your screens are now filled with:
Don't worry, it's not the end of the world. Well, not yet. But it's getting close. So, what can you do?
1. Turn it off and on again. Yeah, that's right. Like your computer is a toaster or something.
2. Update your graphics drivers. Because, you know, maybe it's just a little bug.
3. Call in a professional... or just Google it.
4. Accept that you'll never see the 4K resolution you paid for.
And if all else fails: