How to solve "blocked plug-in" issue?

I use the program all the time, but today when I opened it there is a little rectangle in the middle of the screen area where the flip book normally is and it says Blocked Plug-in. I have not changed any settings on my computer since I used the program last. I don't understand what the problem is.
I rebooted and re-opened but same thing. Can't open any flipbook files. Can you help?

You should update Adobe Flash. Just download and reinstall the latest version of Flash Player.