Slide the keyboard ribbon out of its connector.

If the keyboard ribbon won't slide out, make sure that the black plastic flap holding the ribbon in place is vertical.

It is common for the small ribbon just beside the keyboard ribbon to wear out with use in this model. (Keyboard and trackpad would not work but power button will.) The solution is to apply pressure directly on that cable where it enters the flap (with a piece of plastic or a folded paper that is pushed against by the battery).