The itunes volume is currently turned all the way down - Cannot click OK to open iTunes??

Hi,

I am running OSX Yosemite and today opened itunes which is in Full Screen mode and I get the following message :

The itunes volume is currently turned all the way down.

To change the volume, drag the slider at the top of the iTunes window.

Then an option to click OK.

But I CANNOT CLICK OK. I cannot get it out of full screen mode, I can only force quit ?!?! Any help please. This is driving me nuts!!

Thanks in advance 🙂