Keyboard and trackpad not working, SMC reset seems possible

There's a very simple way to see if this is a software or hardware problem. Power on your machine and hold down Command-V. This is the shortcut for verbose boot. If the keyboard works at boot, but not at a login window, its software. If you cannot perform startup commands properly, it is a hardware issue, as no software is loaded this early in the startup sequence.

If it ends up being hardware, possibly a poor cable connections has caused it since the machine was opened recently.

In my case my macbook's keyboard was disabled after being connected to a miscellaneous bluetooth keyboard somewhere in my house. I used keyboard shortcuts to get to System Preferences and disable bluetooth and fixed the problem that way.

If your computer is connected to both a trackpad and a keyboard via bluetooth, you might alternatively have to locate that bluetooth device and use it to disable bluetooth and/or remove one of the devices.

Lastly, a solution is to move the laptop out of range of interfering bluetooth device, then turn off bluetooth once you have regained control.