Add water to your boiler

I had the same problem. One section of the house had no heat and the other two sections are running fine. The plumber just turned on the water valve and added more water to the boiler and the problem is fixed. He told me that my water valve supposes to turn on itself when the boiler water level is low. But somehow it did not. That was $125 problem.

Also make sure your pipe has no air in it. search bleed air from boiler water pipes.

Good luck