iMac late 2009 Thunderbolt hardware not detected

I recently decided to connect my 27-inch iMac late 2009 operating El Capitan for use as a second screen with my 15-inch MBP mid-2015 operating El Capitan. I had a thunderbolt to HDMI cable to connect the iMac to my TV, the last time I remember using that was mid to late fall. The iMac has all the updates installed.

The iMac system recognizes the USBs and the Firewire 800, but it is not recognizing the hardware for the thunderbolt.

Any explanations why the system stopped recognizing the hardware? Is there a way to fix it?