Power button not working

I've yet to see a bad MacBook Pro power button that wasn't caused by moisture (water spill, etc). More often than not, there is oxidation on the power button switch that's causing the problem. Sometimes this can be cleaned off and made functional again but I prefer replacing the switch completely. The good news is the part is relatively easy to get and isn't that costly (about $10-$15). The bad news is to repair or replace the part requires pretty much a full disassembly of the computer. Not an operation for the faint of heart.

As a workaround, you can remove the lower case and disconnect your battery. With it disconnected, plug in the AC power adapter and the computer should turn on by itself.