As of 21 April 2015, OS X now blocks any version of Adobe’s Flash Player prior to version 17.0.0.169, or 13.0.0.281 if you are still running with that series. Those versions are the latest currently available.

So if you now discover that your Flash has suddenly been disabled, you will need to update before it will work again.

You can check your installed version either using Flash Player’s popup menu (when over Flash content), or in the Installed Plugins item in Safari’s Help menu, etc.