Can't force quit even by keyboard

My iMac 2009 (yosemite 10.10.4) is hung up or frozen.

I have the app store app on top of a website so I think it's the app store that is stuck. I also have a website and calendar open. I can't Force quit anything and even tried by keystroke. Also tried just quitting app by Command q but it didn't work either.

Is it safe to turn it off by the button in the back. Just worried about it screwing up Photos.