a/c won't work- breaker in electric panel is tripped and buzzes when you try to reset it

Turn breaker to off position and then turn back on. Breaker should either stay on or it should trip off again. It should not buzz. Bussing indicates that breaker is either defective or that there is a loose wiring connection, more likely loose wiring connection. Turn breaker off and make sure both wires are tight under screws (assuming 240 double breaker). If breaker is 120 single breaker make sure wire to breaker is tight and also make sure neutral wire of that cable is tight at neutral buss.

Could be a bad breaker,a shorted wire, grounded contactor or a bad AC. Usually, the unit just won't operate so I'd lean toward the breaker/wire/contactor. Have an electrician do a little investigating.