Hello my a/c unit just started to blow hot air. Could this be the contactor?

Is is set to "Heat" Just kidding.
I would go outside and see if breaker is tripped.
The contactor gets its 24 volts from Air Handler transformer, after going through thermostat.
If you have 24 volts going to contactor coil and it doesn't energize(pull down),
Bad contactor, Also contactor contacts important.
If it energizes. Make sure 240 in and 240 volts out.

Could be contactor.But first, does outdoor compressor and fan come on. Both come on or just one of them? Is the inside coils frosted up?

Give a lot more info and we can try to help. Heat pump, gas heat, make of unit, is the outside unit running?