I had this same problem and solved it like this: I normally connect to my second display with a USB C cable. It was this display that could not be detected. I got an HDMI cable and with an adaptor connected it to my iPad. Having a second display connected meant that when I was in System Preferences - Displays, holding down the option key resulted in the "Night Shift" button finally changing to "Detect Displays" and clicking this button resulted in the monitor connected with the USB C cable appearing. Apple should update the OS so that "Detect Displays" is always available.