回答“Low light or no backlight on LCD”I had this very same issue happen to me. I have a 2014 5k iMac and 2 x 27" TB displays. Here is what fixed it for me. Unplug the power cable from the back of the display, wait a few seconds, then plug it back in. It has been working fine now for about 4 days so far. Hope this helps someone else.