heater core replacement

[wvwirenut]

I have an 02 250. This weekend the heater core went bad (anti-freeze all over the windshield and coming out of the vents). I still had some heat, for a while. I loosened the cap on the resevoir to see if that would help cut down on the pressure shooting AF all over the windshield. It was really hot inside the cab for a while. I stopped and shut the engine off for about 20 min and when I started it up again, just cold air blowing. I tightened the cap back up, but it was still cold. Did I air lock it somehow? I'm changing the heater core now, but didn't know if something else is wrong too. Sorry for such a long post.