Hi and thanks for using 2CarPros.
Either the blend air door actuator (which is what I think they replaced) is bad, or it has become disconnected or jammed for some reason. I doubt the heater core has anything to do with this.
If the component has failed, disconnected, or not properly installed, you will not get heat. It is located under the dash on the passenger side of the vehicle. I attached a couple pictures for you to use as a reference. You will need to craw around under there to see if. However, since you just had the work done and it only worked one day, I would take it back.
Let me know if this helps or if you have other questions. Also, once you locate the component, try adjusting the heater in the vehicle and you should feel it working.
Images (Click to enlarge)
Sunday, November 18th, 2018 AT 6:52 PM