I had a problem that my screen was very dark when on battery, ie not plugged in   Despite exhaustive searching on microsoft, dell and throughout my computer, I was unable to find a slide bar that nearly every support blog says is there!  Finally, after nearly a year of frustration, my IT contractor told me to do the following:  immediately when powering on the computer, begin tapping the Fn F2 key.  Continue this for about 5-10 seconds until the screen shows up with a brightness bar that you can then adjust.  So simple, yet nowhere have I seen this posted. Hope it helps others!  (Inspiron 5000 series)
I have this issue on my Dell Inspiron 13 7000 series for very long time. I tried searching Google and Dell support community and I did: Control Panel > Device Manager > Display Adapters > Intel HD Graphics 5500 > Driver > Update Driver > Search Automatically. It fixed. 18:35 on 25-Dec-2016: Sorry bro, I did as above; it fixed but when I try pressing the brightness button then It come back problem again: can not adjust the brightness. Hic hic 19:00 on 25 Dec 2016: I even try downloading and installing the latest Intel Graphic Driver (issued on July 2016), but just after a while, the screen comes back very dim. I accidentally discover this: Setting --> Display --> uncheck the box "Adjust my screen brightness automatically". Sorry, but stupid Windows!!!!
Cannot adjust screen brightness on Dell laptop? Simple solution.