MacBooks will no longer detect external displays.

I have been using an LG 4k monitor as my external display at work for about a year. Suddenly it won't work. The Macbook doesn't even detect the display. It won't even detect power from the cable that came with the display. The very same one I've been using for the last year. I've tried a lot of things to isolate the problem. Some notes on that:

• The display works. When I plug in HDMI to use it as a second monitor to my personal PC, it works just fine.
• I've tried using an HDMI with the HDMI to USB-C dongle that you can buy from Apple, still won't detect an external display like it did before. I was inclined to think something was wrong with the USB-C Thunderbolt cable I had been using for the whole year, but again the HDMI approach doesn't work either.
• Both my and my wife's Macbooks will no longer detect the display.
• I've tried all the ports on the MacBook. They seem to be working fine. They all work with my power supply. I've plugged my HD webcam into them, and they work. So the ports are working on the Macbook, just not when I plug the display in. It's like nothing happens.
• I tried connecting my 32" 2k Dell display to the Macbook, as well as another 27" Dell, via the dongle to see if my computer would detect it, but it didn't. So the Macbook isn't detecting any displays.
• I've tried doing the thing where I go to settings > displays and hold the command key to get the "detect displays" button to appear. Nothing there.
• I've done plenty of restarting Mac, restarting display, etc... nothing.

It feels like the problem is software. Like something on the Mac end that simply won't recognize that anything is even plugged in.

I'm running the latest version of Sonoma (14.1.2)

I've done a fair bit of searching and everything recommends solutions tied to things I've already tried.

Does anyone know anything here? Is this a Sonoma thing? I did see some people suggesting this problem was happening with Monterrey, but again I'm using Sonoma.