Streaming from iMac to ATV 3 has stopped working ..

All of a sudden, after two years, my apple ran home ssems to be failing me 😟. I will first mention my issue, followed by my current setup and also what I have tried to do to fix the issue.

I have always used ATV and have been a huge advocate of it while discussing tech with friends and family. I have teo units in the home, one in the living room and one in the main bedroom.

All of a sudden, when I set my iMac to ATV, my just goes black but I can still hear sound through it. Almost as though it's fully connected but I'm only getting audio and no video. It's wortu noting that this has happened with both of my Apple TV units. Here is a list of what I'm currently using in a bid to help dual down to issue as swiftly as possible:

27" iMac late 2013. I5, 8GB Ram. running Yosemite which is fully updated as of today the 2nd of July.

2 X ATV 3 fully updated as of today 2nd of july

aiport Extreme base station

I have tried swapping the HDMIs although I was confident that this wouldn't recrify the issue as when I turn on the ATV (independant of streaming) it has both audio and video.

****important**** the following devices in our home connect faultlessly with our ATVs giving perfect audio and video.

iphone 6

iphone 6plus

iphone 5s

ipad 2

iPad Air

ipad mini (1st gen)

this his leads me to believe the issue lies within the settings or files that are on my imac but I'm at a loss as to what?

Any help on this would be fantastic so thanks in advance!