I believe this is new to the 9.3.0 build 4170-04 release that was pushed to my device at the beginning of the month. This didn't happen before the update, but now it's common to turn on the TV (after not using it for a day), selecting the right input, and having the TV detect no signal. I used to just turn on the TV and the roku screen would show. My work around is to
press home or any other button on the roku (the white light flashes)
turn the tv off
turn the tv on.
sometimes this doesn't work and I need to reboot the roku. It's like the Roku puts the HDMI output to sleep, and it can't wake up until it's connected to the TV, and then the TV resets the connection. I just found the control tv with roku setting, and I've disabled it to see what happens.