I had lost my number keys 1-9, so I removed the lower case, peeled back the protective tape from the keyboard ribbon cable connector, flipped up the ribbon cable retaining flap, pulled out the ribbon cable, cleaned out the keyboard connector socket from both sides with dust-off, wiped the ribbon leads with a screen cleaning cloth, sprayed the ribbon leads with the dust-off, put it all back together and everything is back to normal. I followed steps 1, 2, 24 and 25 from the battery replacement guide provided by ifixit: [guide|89197]