My furnace comes on but no air comes out of the vents

If the furnace heating up? If so the blower motor or blower motor circuit is at fault.

Furnace is on... blower is on.. air is not coming out of wall vents

If the blower is running and no air is coming out the registers it might be a belt drive blower and the belt is broked or slipping. A very dirty air filter will also cause this problem. A dirty A/C coil will also cause this problem. Anything that restricts air movement can cause this.

The professionals just installed a payne thermopomp in my house, after they left I noticed the air flow in the vents became very low, it was very strong before, what could it be, I checked all vents and all are open.