With the door open there is a plastic lock out switch for child safety. There should also be a little decal showing the position for it to be locked and unlocked. If the plastic lock, has a flathead screw driver top, is turned the wrong way your windows will be locked. (maybe this lock is for the door lock not the windows?)
I have a 2008 Denali, I just found out last week that both of my rear windows would not roll down from the back seat switches? I assumed my driver door controller was not working. Took it apart and cleaned it, no change at all?? I took the rear door switch out on one side, opened it up and cleaned everything well. I really did not see anything that I thought might cause lack of function. Put it all back together after cleaning and drying. Switch works like brand new? Did the other side rear window switch and it also works like brand new? I cleaned the electrical contacts with alcohol and a q tip, nothing special. Washed out the switch case in dish soap and water, rinsed well and dried with my air hose. Both rear windows now work. I never would have thought both rear windows could fail like that and be repaired with this cleaning. Worked for me, 08 Denali, 140,000 miles on it. It is difficult to open up the switch once removed without breaking anything. I have a tiny screw driver that helped open the locks without breaking them.