Why won't my iMac detect TV?

I have an iMac running Mountain Lion. My iMac has a Thunderbolt port. I am trying to connect to my LG TV via a MiniDisplay Port to HDMI cable. I have plugged the two together, the TV shows detection through the HDMI port however when you switch the input to the HDMI port on the TV it does not connect to anything. So the TV detects the cable plugged in but can't connect to it, which leads me to think the issue is with the iMac instead of the TV. When I click the 'Detect Display' button in System Preferences literally nothing happens.

I'm not sure where to go from here, I have been looking online but nobody seems to have a definitive answer that works. Some suggeset it may be an issue with the TV's settings, but I have looked and can't seem to see anthing (but I am open to being wrong). If anyone has any answers then please let me know!

Thank you.