You probably shorted out the backlight circuit. that's what the spark was. You have to disconnect the battery before disconnecting or reconnecting the screen assembly otherwise the backlight circuit has a tendency to blow. On the LCD connector, VCC_MAIN and GND are adjacent pins so if the connector is not perfectly lined up, it will short.
If your lucky, it's just a filter however sometimes the damage goes upstream and blows thew diode and or driver IC. This requires a micro-soldering repair.
To be certain, connect your phone to an iTunes enabled computer. Shine a bright flashlight on the screen and if you see a dim image, the the problem is the backlight circuit.