external monitor not recognised or...

I use a 22 inch monitor, connected to a 13 inch laptop by a cable. Windows 8 filled the 22 inch screen, and I was able to deselect the 13 inch one if it became a distraction.
But Windows 10 is showing the 13 inch screen image at the same size on the 22 inch screen - i.w. not filling it, and when I found my way to <display> it seemed that it regards the system as having only one monitor , and when I asked it to search for the other it couldn't find it.
I know there is some simple trick for this - I did try Windows 10 a while back, and someone solved the problem then ; but I went back to Windows 8 - then decided that I wanted another try at 10.

 

Hi Geoff, Try > Settings > System > Display > Identify/Detect. Neil.