Help with diagnosis

I have a lennox unit about 15 years old and for the first time my outside unit wont come on. The thermostat appears to be working (blower comes on after setting the tempature) circuit breakers and wiring appear to be ok. What i did find was there is a relay/switch box on the outside unit that i can push and turn the system on manualy every thing works ok. There has to be something that tells this relay/switch to turn on. Any ideas about this problem? Just trying to save some $$$$$$$