Unable to detect monitor

[NBurns2020]

The old Hyundai L72D monitor with both DVI & VGA input's has worked quite happily as a monitor on my system.
And it worked perfectly well when I was mirroring my MacBook Pro (2011) output to this monitor running High Sierra 10.13.6.
My other external monitor an Acer AL1916W monitor with just a VGA input I've been using perfectly well as my other external monitor extended to the right of my laptop.

Initially when I looked for 'extended display' it would not show in the software.
Now, the monitor is not working at all.
Help please?
If you need any information and log files I can supply them if you tell me where to look.

Background was in Telecomms engineering but this has foxed me today when my DisplayLink just arrived.