Laptop turns on, but no display to screen or external monitor

Model is Toshiba Satellite S55t-B233.

The laptop turns on and stays on, but the LCD never turns on (and stays on) and it doesn’t put display out to an external monitor via a working HDMI cable. I have tried replacing with working RAM. I have already tried 2 different motherboards and unplugging everything else, but there isn’t any change. I did some testing with a multimeter just to be sure and enough power is getting to the motherboard.

Thanks in advance.