I have a Dell Laptop (that I can dual boot into Ubuntu or Windows and has an HDMI and USB-C port) and a Brand New (got it yesterday) Dell Monitor. When I plug the monitor from HDMI 1 into the HDMI port on my laptop and everything worked perfectly.
This morning I plugged in a "raspberryPI-like" device into the HDMI 2 slot of my monitor. I've had trouble with this device over the last few days, so it didn't surprise me that it didn't work. However, the moment I went to switch back the monitor input to HDMI 1, the signal to the laptop stopped and the monitor "disappeared". No amount of unplugging/rebooting made a difference. Plugging any HDMI cable (I used a couple) from the monitor from either HDMI 1 or 2 into the HDMI port of the laptop didn't register a new monitor. I tried this on both Windows and Linux and the behavior was the same.
I did find that if I plug the monitor into a USB-C hub with an HDMI port, then a monitor was detected and it came up (for both Windows and Linux), however on Linux it says it's an Unknown display and cannot get the HD resolution that I could before. I've got another laptop without an HDMI port, and confirmed that it worked through a USB-C/HDMI adapter, but not through the HDMI port on my docking unit.
I also have another monitor (different model) that I confirmed worked when plugging into the HDMI port of my laptop.
So I've checked and used multiple cables, I've confirmed the problem exists on multiple OS's, and I've confirmed that the laptop works with other monitors.
Any ideas as to what may have happened? I'll also add that the reason I have a new monitor is because the old one (which was the same model) "stopped" working two days ago after working perfectly for 9 months. But it also stopped working right after I tried using that same "raspberryPI-like" on it (though because I was doing a lot of messing around with my different peripherals, I didn't correlate the two yet)
I don't see any dmesg's when I try plugging the monitor into the port. Any other ideas for seeing what errors the kernel may be seeing when I plug it in that could help identify the issue?