1992 Chevy Cavalier Heat Problem?

I have a '92 Cavalier. The heat had been working fine. I shut it off, and I started it up about 30 minutes later and no heat ever came. I have replaced the water pump because I was told I wasn't building up any pressure in the cooling system, and I have replaced the heater core as well since the water pump did not fix it. The thermostat is no more than 2 weeks old, I changed it with the upper radiator hose since they are both there together, and the hose had been leaking. At any rate, I still have no heat in my car. I'm freezing to death! help!