I have a carrier heat pump. It was installed in 1991. The inside unit is blowing out warm air. The out side unit is making a humming noise, but the fan is not turning. The outside air temp last night was about -5 degrees. Any ideas what may be the problem? Could it be froze?

Some equipment will shut down the outdoor fan & allow the compressor to run under certain conditions. It would be worth having it checked. An experienced service person could tell you by looking if your equipment is set up that way.