I'd load up Health Check and see what it's reporting. The drop test may be a detect on the power button or the slide-lock button. I'd bet that the alignment in the rail sensors is off just a hair now and it's not picking up the magnet. (The rail detects are magnetic, which is why a magnet positioned just right can cause the device to unlock.)

It could also be something as simple as a connector being jarred slightly out of place. If you're comfortable opening it up (apparently you are), I'd say open it and poke around. If it's just a loose connector, the act of opening it and putting it back together could fix it.