I have a 13-inch early 2015 Macbook Pro with Retina display, and today the keyboard and trackpad stopped working completely. (I have OS X 10.11.5 if that's relevant. I can't update to sierra, as a software I use is not compatible with it). The only thing that works is the power button. Now, this isn't unprecedented. This started happening a few weeks ago, but every time it happened, shutting down and restarting by holding the power button fixed the issue. But today it didn't.
I know there are many questions like this, but I tried every single solution I found. This probably means I won't get one here either, but I want to try this before I have to go to the Apple store.
I plugged in a USB keyboard to my laptop so that I could use it. I also plugged in a USB mouse, but that mouse plugs into the keyboard, not into the computer. Both worked fine. I tried an SMC reset and a PRAM reset, using both the built-in keyboard and the external keyboard for the required inputs, for a total of 4 attempted resets. I can't really tell if they did what they were supposed to or not, but they didn't fix my keyboard.
I booted in safe mode (using the shift key on the external keyboard. Using the one on my laptop didn't work, and it just booted normally), and the issue persisted.
The other solution I saw was to disable FileVault, but it was already disabled prior to this incident.
I also saw people saying plugging in a USB drive during login fixed the issue, but this didn't work either.
The last solution I tried was to let the computer sit while powered off and not connected to the battery. This didn't do anything either.
I did not spill anything on my computer prior to this incident, so I have no idea why it started happening. If anyone has solutions for this that I haven't tried yet, please let me know.
EDIT: I just noticed that if my computer is asleep, clicking the built in trackpad wakes it up. However, the trackpad doesn't do anything else.
Best Answer
You need to replace your top case assembly.
Your keyboard and trackpad (built in) are USB devices, so when you plugged in your external mouse/keyboard and everything worked, you effectively ruled out software issues. This is also confirmed when trying to boot in Safe Mode or doing NVRAM/SMC resets via the built in keyboard.
If it doesn't work in a pre-boot environment, it's not going to work at all.
The only option left is replacement of the top case assembly itself (Apple PN# 661-02361) and given that both have failed, it's highly likely that the USB interface on the keyboard has died.
Technically speaking, you could replace just your keyboard and trackpad, but it's an extremely labor intensive proposition. Whether you are replacing the keyboard/trackpad or the assembly itself, it will require a complete disassembly of the MacBook innards. iFixit.com has a writeup on the procedure and they rank it as "Difficult" with an allocation of 3 hours time.
Even if you don't plan on undertaking that repair yourself, I do suggest you familiarize yourself with the procedure to understand what's actually involved. Suffice to say, it will be an expensive repair.