Keyboard and trackpad stop working after I log onto my macbook air

I have a macbook air and I've been using it for about a 3 years, I was using it then all of a sudden it turned off.

I was able to turn it back on and use the keyboard and trackpad to log on but they immediatly stop working one I log on.

I've tried several things including

booting into safe mode

resetting SMC and PRAM

deleting .plist files in the library

and I even took it to the apple store and they didn't know what was wrong with it (they offered to send it off for 250$ to check it out even more)

none have worked so far and at the moment I am using an external mouse and keyboard

been trying to solve this problem for hours

any help is greatly appreciated