Heil condenser unit will not turn on?

My outside condenser unit looks to be a bit old and now the other day when I turned the thermostat to cool, the air handler will blow but the condenser outside will not come on. This is a residential unit and I have checked to make sure the unit had power at the panel on the unit. Is there a way I can check to... show more My outside condenser unit looks to be a bit old and now the other day when I turned the thermostat to cool, the air handler will blow but the condenser outside will not come on. This is a residential unit and I have checked to make sure the unit had power at the panel on the unit. Is there a way I can check to make sure it is getting the signal needed to turn on from the thermostat inside or can anybody advise me of my next move? It was operating perfectly fine and blowing cool as usual just days before.
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