I would "Extend these displays", OK my way out, reboot and try again. Be sure to shutdown and pull the power plug on the PC to make sure it resets.
I have two displays properly hooked up two my desktop PC. Until an hour ago, both were working fine; that is, just as I wanted, I used to get my browser web pages on Display 1; and my Thunderbird emails on Display 2.
I wanted to have a slightly larger font size; so I went to Control Panel > Display, and made the font size change. But then I must have inadvertently done "something" which resulted in the situation that I can no longer get my browser web pages on Display 1, and my Thunderbird emails on Display 2.
To fix that problem, I went to "Customize Your Display" page. At present it shows only one of the two rectangles "1" and "2" highlighted, but not both "1" and "2". There are 4 options in the pop-down menu for Multiple Displays:
(1) Duplicate these displays
(2) Extend these displays
(3) Show only on 1
(4) Show only on 2
I would have expected another option, maybe titled "Show both displays, selection of which would result in highlighting both rectangles "1" and "2", so that then I could get, as I wanted, my browser web pages on Display 1, and my Thunderbird emails on Display 2.
In the absence of that fifth (or more) option(s), no matter which of the above four available options I try, I cannot at all get what I want. Thus, depending on which of those 4 options I select, I get either identical views on my both displays or one of the two displays remains blank as if I had only one display hooked up where I can get EITHER my browser web pages OR my Thunderbird emails according to my choice.
I have no idea what I had done in the past in Control Panel > Display to properly obtain my browser web pages on Display 1, and my Thunderbird emails on Display 2.