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.