My father bought the TCL 55" Class - 6-Series - 4K UHD LED LCD TV from Costco in December 2019. It was working great until mid-May when he had the same issue everyone is describing in this forum: no picture (black screen), but the sound works when on HDMI1 (connected to the cable box). The Roku home screen did work, however, and we were able to watch Netflix. I tried all of the solutions suggested in this forum (thank you!!), but they didn’t work for us.
Here’s what worked for us—turn off the TV and cable box. Unplug the power cord to the cable box, wait 30 seconds, plug the cable box back in, turn everything on. There must have been an issue with the connection between the cable box and the TV.