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* Turn the computer and monitor off
* Disconnect all cables from the monitor including the power cable
* Press the monitor power button in for 15 seconds
* Reconnect the monitor cables to the monitor and the computer video card. There should only be one video cable connecting the monitor to the video card. Tell me what video card out port you are using to what cable to what monitor video in port
* Turn the monitor on, then the computer
* Press the monitor Menu button to open the monitor OSD (On Screen Display)
* Go to Input Source and check the correct one.

Thanks XPS_Man - I've tried your method removing the power cable and the only video cable (HDMI, connected via a port replicator which my Latitude 7200 connects to) but every button I press still only brings up an option to choose between HDMI or DisplayPort.  Just can't seem to get it back to the factory setting.

First 2 steps are very important 

Disconnect every cable from Monitor and hold the power button for 15 seconds. This will release any residual power from Monitor

This worked for me.