Loss of Heat

[95greensvx]

I am assuming that my heater core went bad but hoping it's something much easier. My 92 sat for a few weeks before I was able to acquire the small part I need to fix the shifter assembly. I started it up, let it warm up and everything worked fine, blew out warm heat and all was well. I then drove down to forked river, new jersey for my work for this week. It was a 4 hour drive and the heat worked the entire way down. The next morning, yesterday, when I got up and warmed up the car the heat just never decided to work. Is there anything I could look for to diagnose the problem or does it sound like a heater core malfunction? Thanks in advance.