It's possible you shorted something by not removing the battery. In general, even if the guide doesn't say it, you want to make sure there's no power to the electronic components. Powering the device off isn't enough since residual power can still be supplied by the battery.
One way to rule this out is to turn on the computer by pressing the power button and connecting the device to an external monitor. If the external monitor shows a signal then you probably just have a bad display or something related to the connector on the board. Another way might be to reinstall the old screen if you still have it handy and the display is actually working.