Heating pipes that have air in them need to to be bled out. The process is called bleeding the pipes, and it is a common occurrence with virtually every hot water type of heating. Heating pipes with zone valves and hot water radiators will all, at some point, get air bubbles in the pipes. These bubbles act like little water dams and will not allow the heated water to pass. When hot water no longer passes through heating pipes, bleeding them is the only way to restore the flow.
Turn off your boiler and allow the system to cool for at least one hour.
Turn the bleeder valve open by either attaching an adjustable wrench to the nut, or on some heating pipes like radiators, use the finger twist turn knob or key. Turn it in a counterclockwise direction to open it up.