I need advice

I have a High Velocity Furnace.

2 weeks ago, it stopped blowing heat. I had a guy out and he found that the pump that moves the hot water through the system was dead. So it took a few days to get the pump, he installed it and I had wonderful heat.... for 24 hours. Then the heat stopped again. The first thing I thought was that the pump was defective and died. But the pump is working fine, there is plenty of hot water. too.

So just what else can be going wrong? I'm going to have the guys out again, but I would like to understand ahead of time what might be wrong. Especially after the fact that replacing the pump did actually get us warm air again.