Dell monitor no longer getting signal from Mac?

[ApplePie22]

I have a 27" imac flanked by two additional 27" Dell monitors. Turned on the computer last night, and one monitor stayed off, said no signal. Weird. So I went to go to the computer settings and see if I found anything, I got a message I've never seen before "Preferences Error - Could not load Displays preference pane." Eventually that worked, and only two monitors showed under the Display section. Weirder. I finally assumed it must be a bad DP cable, swapped with the other monitor, and nope, it works, so the cable and Lightning port are both good. The only thing I haven't done is checked the monitor with another computer, as I don't have one. So my guess is either something is buggy with the Mac, not sending a signal to the second monitor, or, the monitor itself is bad....any guesses?

 

[meson]

I would start by shutting down the iMac. Unplug both external monitors. Then restart the Mac. Open displays preferences. Connect the monitor that is not working. If it comes back, you should be good. If not, look to see if you see the detect displays button. If you don’t see it, hold down the option key and see if it appears. Hopefully pressing the detect displays button works.

If that doesn’t work can you try a different input on that display? Hdmi or even dvi or vga depending on its age?