Having no heat isn't usually a sign of the heater core needing to be replaced. The heater core pissing coolant into the cabin is usually a sign of that.
If your not getting any heat, the likely suspects are:
1. Thermostat stuck open, temp gauge is not an indicator of this as the engine can run 20-30 degrees cold and the gauge will still be in the middle. Feel the upper radiator hose while the car warms up in your driveway. At some point, the hose should suddenly get quite a bit warmer, and the radiator fan should come on (this will take a while with cold outside temperatures.) An OBD 2 reader/dongle that allows you to look at ECU data will allow you to look at the coolant temps as well.
2. Air bubble/air lock in the cooling system. If your cooling system was recently opened up for any reason, possible it wasn't bled properly.
3. Low coolant level.
Only after you've checked those items would I consider a blockage in the heater core, as typically you'll get a blockage in the radiator first which would manifest itself as hot running in the summer.
