Of course this could be 100 different things.
Any way, if the heater is not blowing heat it usually is because the fluid level is low or the heater core is clogged. If I had to guess from every thing you have stated I would guess your cooling system is clogged or restricted both in the radiator and the heater core.
A quick way to tell if the heater core id clogged is to fully warm the car up and turn on the heat. Touch the two heater hoses that go into the firewall. If one is very hot and the other one is not hot the heater core is clogged. I would also assume that the radiator is clogged.
Hope this helps
Saturday, January 14th, 2006 AT 5:14 PM