I just spend an hour or so trying to figure out how to access Joplin on my macbook after having unplugged my external monitor.
As I am still at home, I could have connected the screen again, but this time I thought to find a way without this easy way, because it may happen when I will be out for the day.
First I checked some basic stuff like Force quit, Dock Options >Assign to Monitor 1, ... then I found the following article here in the Forum, but is focused for Windows:
Using Multiple Screens (Joplin Screen not showing after removal of large screen)
Next I went through all of the suggestions of a superuser.com page:
Move an off-screen window back on-screen on Mac OS X
... up until:
option + click the app icon in the dock two times and it will first minimize, then restore the windows into full view on your desktop.
Not sure if the above will work in every situation, but the above page shares as well 20+ alternatives.