I switched to heater for few days and now the AC does not work.

Under what circumstances does it not work? Is it freezing outside? If so the system might not activate to prevent water vapour freezing up and damaging the condensor. Also, did you have the system serviced at all? A low system pressure will also prevent the A/C from being turned on.

Thanks Lars. But outside climate is hot enough and AC is required. It was working fine just few days back until I switched it to heater. The compressor clutch does not activate as it does not get the power . I checked it with multimeter. All the fuses are ok. I appreciate your help.

Have the A/C system serviced to make sure there's enough refrigerant in the system for the low pressure switch to enable the clutch to be engaged. Nine times out of ten that's the problem (unless there's a problem with the switch itself).