Does your temp gage come up when the car warms up? If not it could be a bad thermostat.

If it does and you get no heat out of it, you may also want to look under the hood, there is a small valve that is controled via cable from the heater control. If that has broken, or come off the valve, it could render the valve unmoveable and not give you any heat. The heater core, from what I remember is a PITA to change. I believe you have to take the ENTIRE dash out to replace it.