Why doesn't the overdrive button in my Daewoo Lanos respond?

I drive a lanos with a 4 speed automatic transmission. For some reason the power/overdrive button has stopped responding. Any ideas?|||bad thermostat|||What gear are you in when you press it. If your in second it wont do anything till your in 4th. Or is the light on the switch not coming on either.|||I don't know war that other guy got that answer. I would replace the switch