Clean up the contacts all around but after this amount of time, there is a chance the chip has lost its programming. Also, squirt some contact cleaner into the switch. It's possible you just got some corrosion in there. Wiggle the button around a bit while you do it.
Pull the tail spring and check for corrosion between it and the body tube.
I have an extreme III that was sitting idle for a long time. Tail corrosion was what kept it from running.