I had the same problem. It started when I forgot my charger at home and borrowed a MacBook Air charger. A day later the keyboard and trackpad had stopped working. I used an external keyboard and mouse until I have drained the battery. I opened the computer and tried clicking the trackpad cable in place. A day later I got back my charger and the the keyboard and trackpad worked like a charm again.
So I do not know it the problem was me charging the computer with the wrong cable or if the cable had been misplaced or a combination of the both.
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EDIT: Since I wrote this post the keyboard and trackpad have stopped working a few more times. But now I have found the solution to my problem. I am pretty sure it is the trackpad connector that there is something wrong with. Disconnecting and reconnection the connector (see step 17 in this guide: https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Retina+Display+Early+2015+Trackpad+Replacement/45138) have fixed the problem. I have also bought a new cable and will install that in my computer when it arrives.
I had the same problem. It started when I forgot my charger at home and borrowed a MacBook Air charger. A day later the keyboard and trackpad had stopped working. I used an external keyboard and mouse until I have drained the battery. I opened the computer and tried clicking the trackpad cable in place. A day later I got back my charger and the the keyboard and trackpad worked like a charm again.
So I do not know it the problem was me charging the computer with the wrong cable or if the cable had been misplaced or a combination of the both.