I can confirm that this has happened to me too.
joplin stopped working on macOS after updating to Catalina 10.15.2
After working properly on Friday, on Monday(today) all of the sudden Joplin is not working anymore.
Not Working means: Joplin Starts, but it is not showing any windows.
So now after reading the above post, I paid attention and I saw a 1 pixel window at the bottom right corner. I managed to drag it to the center of the screen and to enlarge it.
Now everything seems to work just fine.