This just happened to me last night:
My laptop was running on battery, and the charge dropped to low levels. Naturally the screen brightness began to dim as it was told to. It got pretty dark and hard to see, but that is probably due to a bad setting somewhere.
Here's the unexpected behavior: When I was done working for the night, I shut my computer off and plugged it in to charge overnight. This morning the battery was fully charged and the computer was plugged in. However the screen brightness did not return to normal settings!.
Once all the drivers loaded I was able to use my Fn keys to increase brightness. However that may not always be the case. I would expect screen brightness to return to normal, especially when the laptop is plugged in. I could not see the grub menu, for instance, and I can't use my Fn keys until the system finishes booting.