[AT&T LG Optimus G] SevenGPLuke:
This is the fix. Or should I say "Completely Stupid Time Consuming Trick" that somehow fixes the problem(at least for me) because now my phone powers off and on normally. Here the Steps I've done. I tested this out 4 times and it worked all four times but it was only tested on my device. So this is not guaranteed to work and its time consuming.

Step 1: Plug in charger and let it sit there at the black screen for 7-10min

Step 2: Plug out charger and hold down power button until phone fully powers off

Step 3: Hold Volume up and down at the same time and plug in charger. It should be in download mode. Let it sit there for 7-10min.

Step 4: Repeat Step #2

Step 5: Plug in charger and wait about 20-30secs. :fingers-crossed: If all went well it should go to the battery charging icon or menu or whatever you want to call it. If it does, let it charge for about 5min and fully power it on.

Congrats your phone is now on and past the black screen. :good:

This will temporarily fix the black screen problem. Well... not fix but at least get past it and have your phone on. I suggest you never fully power off your phone and use power off and restart in power option instead of power off. To fully fix it do the following Steps:

Step 6: Power off phone by holding power button until power menu pops up, and by selecting power off. When phone is fully powered off, plug in charger and wait about 20-30secs and see if it goes to the battery charging icon or menu or whatever you want to call it. If it does then proceed to Step 7.

If it does not and just sits there at the black screen, you will have to complete Steps 1-5 to get you phone back on again and retry Step 6.:crying:

Step 7: Let it charge for about 5min and fully power it back on.

Step 8: Power off phone by holding power button until power menu pops up, and by selecting power off. Now power on phone again and see if it powers on WITHOUT THE CHARGER. If so proceed to Step 9.

Again,:crying: If it does not power on and just sits there at the black screen, you will have to complete Steps 1-5 to get you phone back on again and repeat Steps 6-8 again.

Step 9: Power off phone by holding power button until power menu pops up, and by selecting power off.

Step 10: Hold power button until phone flashes lg logo, then goes completely black(not the dim lighted black screen, COMPLETELY BLACK). As soon as it goes completely black, let go of the power button and wait for your phone to power on.

If you phone powers on it should be fully fixed and should power on and off normally(well at least for me). But AGAIN *sighs* If it does not power on and just sits there at the black screen, you will have to complete Steps 1-5 to get you phone back on and repeat Steps 6-10 AGAIN.

I recommend you just do Steps 1-5 to get your phone on and past the black screen. And to use power saving technique's to save battery when not in use of phone or when not near a charger. I may make an YouTube video on how this "Completely Stupid Time Consuming Trick" is done. But anyways this is what I did to fix the black screen issue. Let me know if Steps 1-5 at least gets your phone past the black screen and on. Steps 6-10 is pointless but worked for me but time consuming.