LG TV turns on saying "Input signal not found"...grrrr

I've reset the TV to manufacturer settings, ensured the TV auto-turns on cable box via HDMI. I've gotten a new cable box and new HDMI cable. I've tried both of the HDMI ports. It works once the cable box is manually turned off and on, but overnight it shuts down again and in the AM, won't work again. What can be done to make it easier for a senior citizen to just turn the ON/OFF button and have the cable TV work?