MacBook Pro 13-inch, early 2015: keyboard and trackpad stopped working, power button still worked. I was able to fix it by using a usb keyboard to do the following steps:
NVRAM reset:
# Shut down your Mac.
# Turn on your Mac, then press and hold these four keys together immediately: Option, Command, P, R.
# Release the keys after about 20 seconds, during which your Mac may appear to restart. For example, you may hear a startup sound more than once, or see the Apple logo more than once.