2006 Ford Focus Heater/Air Conditioning is not blowing at a

My heater recently began making odd clicking noises, and every now and then would shut down, then come back on in a few minutes. I assumed the engine might not be getting enough coolant, so I topped that up and figured it would be all good. However today my heater stopped blowing air entirely hot or cold. I have checked the fuses and they are fine, all other electrical components such as rear window defroster, etc. Work fine. What might the problem be?