If your Flash plugin simply stopped working all the sudden, maybe this can help you.
The message shows: "Couldn't load plugin".
The issue relates to having the adobe plugin installed using the flashplugin-installer package (which is used by Firefox), and the plugin now embedded to Chrome at: /opt/google/chrome/PepperFlash.
The solution: disable the adobe flash plugin in chrome://plugins:
And it is back to normal!