I have only ever had 1 monitor connected to my desktop, but somehow I've managed to get it set as screen #2.

I just had to replace a blown out monitor, so maybe it recognized this wasn't my original monitor and set this as #2 (??)

Settings/System/Displays shows screen 2 as a large rectangle, and screen 1 as a small grayed-out one. The Detect button finds only my single monitor. Under Multiple Displays "Show only on 2" is selected. "Make this my main display" is checked and I can't uncheck it, presumably because there is no screen 1.

I don't know if it's true but I've read there are some programs that output to screen 1 no matter what Windows is set to.

How can I switch the number of my connected screen to #1 (or can I?)?

Does your graphics card support two outputs? If so, did you reconnect the new monitor to the original output?