For some reason, my notepad wouldn’t open, suddenly. The icon will appear on the taskbar, I can right click and access the icon, but I couldn’t maximize or restore. After searching the web, I found the below link. Essentially, you will have reset your monitor to "Computer Only".
I am using a single monitor now. I probably had an extended monitor at some point. Notepad is prbably opening on a non-existing 2nd monitor. After I reset to computer screen only, it brought back that window to my main screen and everything is ok again!