(94 Z28) my car was on ramps for the past month and i finally got it running and my heat was not blowing that hot, and now all of a sudden it blows cold.also my windshield defroster makes a humming noise wen i turn it on...anyone know what this could be?

 

What did you do to the car while it was "up on ramps"? Knowing that might provide a clue as to what happened.

 

i did a new head gasket job.

 

Ok, so you had the cooling system open. Often the coolant will coagulate when exposed to air. Flush the heater core.