My heater stopped blowing hot air

First make sure that your water pump is ok, since a bad pump will cause no heat due to lack of coolant circulation.

Bad sample:
http://www.youtube.com/watch?v=thcwatH_BpI&feature=plcp

Good sample:
http://www.youtube.com/watch?v=SJN3biTfdGE&feature=plcp

The other possibility is a broken blend door lever...stuck in the cold air position.

http://www.youtube.com/watch?v=6U9ZdkAjdhQ&feature=plcp

Look under the dash to see if your blend door lever is attached and moving.

Blend door foam coming off of the doors / flaps will also cause no heat symptoms.

Thomas