I have a bryant a/c unt outside it will not come on , what could be the problem . The conntacor will not stick

If the contactor will not activate it is usually or could be caused by the following.

Defective contactor.
Defective 24 volt source that supplies the contactor
Defective/damaged 24 volt thermostat wiring

Is the inside blower running when it calls for A/C in the automatic blower mode at the thermostat?