Typical unit only shown here, usually located inside the furnace compartment:
These devices detect system faults and can prevent the compressor from running (therefore, contactor not pulled in). It should have either a flashing LED as shown here, or possibly a display showing a fault number. Refer to your user manual for a list of faults. Most faults are for furnace (heating) operation, but yours could have some AC (cooling) protectives as well.
Other than that, if the feeder fuses are good, there may be a resettable overload device on the outdoor unit that has tripped, sometimes with a red reset button. It would be very informative to know if the compressor contactor is energizing when the thermostat calls for cooling. It's possible the contactor or thermostat has a problem. You should see 24Vac on the contactor coil during cooling demand. Be very careful around that device because it will also have 240Vac on it as well.
If the compressor capacitor or windings are bad, the compressor will not start and usually will blow a fuse in the disconnect box, or trip an overload device as mentioned above.
Best of luck!
KY
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