Let's go back to your original complaint, the unit not coming on. When you say unit, does this also include the inside blower that is supposed to come on along with the condenser outside?
If both the blower and the condenser do not respond, you have low-voltage issues involving the thermostat circuit. The thermostat voltage is supplied at the furnace by means of a transformer. Generally there is a small fuse on the newer systems which protect the circuit from overload, so look inside your air handler for this and examine the low-voltage connections at the furnace.
You can test the thermostat voltage at the furnace where it all connects together. The red wire from the transformer is your 24 volt source to the wall thermostat. This must be measured against the common terminal on the transformer to properly ascertain the voltage. At the wall thermostat, you can also check for 24 volts between R and G, for instance. This will use the common side of the fan relay as "ground" for the test. Do you have a fancy digital programmable wall thermostat, or is it the plain no-frills design?