If you have just noticed that Flash has stopped working on your Mac, it is the result of OS X now blocking all versions prior to 21.0.0.226 (or 18.0.0.343 if you are still running that series). This is because previous versions have security vulnerabilities which are being actively exploited, making them dangerous to use.

Overnight Apple pushed this out in a silent update, as it normally does.

You can obtain Adobe Flash Player version from here.