I am running Illustrator 2019 because one of my plug-ins will not run on higher versions. It has been running fine for a while now but today i have a question mark on the icon to launch it and it will not open.
The question mark on the icon generally means that either the application is corrupt or uninstalled. Please verify if the application is indeed installed. You can check so via the Creative Cloud app, it shows all the versions of the program installed on the computer.
I suspect this i with a Mac operating system. I get this everytime there is an upgrade (not just update) of the software. I remove the question mark from the dock, create a new shortcut for the new version of the software.
If the upgrade replaced your old software you can use the Creative Cloud software to install older versions and keep using your plugin.