Why is the left side of my screen dark?

It’s very important to always disconnect the battery before disconnecting/reconnecting the screen assembly. The issue is that the LCD connector has pins that have high voltages (i.e. the backlight is ~20V) and ground in very close proximity. When you fumble around trying to line up the connector plug to the receptacle, you can short out the voltage rail and cause damage to the components in the backlight circuit. If you’re lucky, then only a backlight filter is damaged (there are tow, one for each half of the screen) but other times, the entire circuit could be affected. Either way, this type of repair requires micro-soldering and isn’t really a DIY repair.