So my heat stopped working. Still blows, it's just cold air. I've replaced the heater core and thermostat. Not sure what else to try. Please help!!!

 

Make sure your coolant is topped up. After replacing the t-stat and heater core, it's possible that air was trapped in the system as they refilled it with coolant. The air can work its way out but leave the system low.
Feel the heater hoses. Are they both getting warm? If only one is warm, something is plugged. Hoses have a liner that can detach and block the flow.
Check the functioning of the blend door. The blend door moves to mix warm air flowing over the heater core with cold air from outside to modulate the temperature of the air coming in to the passenger compartment. It is moved by an actuator. If that has gone bad, you won't have control over the temperature of the incoming air.
Make sure you're operating the system correctly with the controls.
If you have ATC (automatic temperature control) your electronic module could have gone bad.
There could be a problem with the control head on the dash.

 

who replaced your heater core?