The reasons can range from your circuit breaker tripping, blowing a fuse, clogged air filter, dirty evaporator coils or condenser lines, refrigerant leak, motor, thermostat failure, or it could be a wiring issue.
Check to make sure your thermostat is set to AUTO. If this is not an issue then the problem may be a more serious one and a professional will need to be called in.
If you hear noises from your compressor or outdoor unit, this normally means your heat pump needs repairing. Electrical heat pump issues can make a popping or buzzing sound. A failing motor can buzz or vibrate. Also, a failing valve or solenoid may make a hissing, whooshing, or vibrating noise.