iMac doesn't recognize external display

I have an external monitor connected to my iMac. Everything was working fine until I upgraded to MacOS 15.4 Sequoia. The external monitor is no longer recognized by the computer. Upgrading to 15.4.1 didn't help.

The Displays pane in System Settings does not recognize the display. Using the option key to bring up the Detect Displays option does nothing. The Thunderbolt/USB4 pane in System Report states "no hardware was found."

I know the monitor is still working as I can connect it to my iPad and it works fine.

I've seen the threads stating the solution is to trash the preferences file for windowserver.displays, but I don't have that file on my computer. Really, I don't.

I've endlessly tried restarting the computer and display, changing cables and ports, and I simply run out of ideas. Would appreciate any assistance getting this display working again.

[Re-Titled by Moderator]