So I took apart my Toshiba Satellite A70 including the screen. When I put it back together now the backlight will come on but no image will display. If I connect it to an external monitor that works. There in only 1 harness to connect the lcd to the mainboard. This should be simple. All I did was unplug the screen, set it aside, and plug it back in.

Does the fact that I turned the laptop on without the screen attached make a difference?

You missed plugging something else in.
Look for the inverter, which plugs into the lcd backlight with 2 wires and a white plastic connector.