I have a older unit and the fan and compressor will not come on but if you push the contactor in it will come on I've replaced the contactor and the starter compacitor and new thermastat any info will help

Pushing the contactor in tell you that you have high voltage, start tracing the smaller tstat wire that runs from furnace to condenser unit for cut wires., many times pets or weedeaters will chew the wires into, if you find the problem turn off the breaker to furnace,so when you splice wires back together you won`t fry the transformer

Start there as wmproop suggested and then look at your wiring from new stat, you may have wired it wrong...

Thanks everyone it was the circuit board

Was my next suggestion... glad your settled.