Unable to adjust the screens brightness

Lets start off trying a SMC reset. Follow this Apple T/N: Reset the System Management Controller (SMC) on your Mac. Let us know if that fixed things.

Well you have me scratching my head here a bit ;-}

I'm wondering if your NVRAM battery is having issues so the setting in NVRAM is not remembered or corrupted do to the power loss.

To test this you'll need to go to the clock settings. Open Date & Time preferences control panel. What we want to do here is alter how your clock works. Unlock the panel and then uncheck the 'Set date and time automatically'. This will force the system to use its built-in clock to keep time (which uses the battery). Save the settings then shutdown your system and unplug the AC cord for a good hour (more if you can).

Now plug AC cord back in and power up the system is the clocks date & time correct? If not the battery is failing! If that doesn't nail it I suspect your logic board is having problems which will require bringing the system into Apple or Apple authorized service center.