Help ! I've got a warning screen that won't Respond or Go Away !

["mentor2@gmail.com" <me...@gmail.com>]

The screen doesn't respond when either button is clicked and now whenever
I want to shut down my iMac *I have to cut the power by unplugging it.* I
have
no idea what caused the screen to initially appear, but it's very
frustrating...
Can you help me ? I've attached screen grab of the Warning...

Thanks,

Bill Gilpatric / Acton, Massachusetts

-- 
Do not wish to be anything but what you are, and try to be that perfectly.
     ~ St. Francis de Sales