I have a High Velocity forced hair heating system that exchanges the air with hot water from my hot water heater. The hot water is pumped through a coil in the HVAC unit by a TACO 006 B4 cartridge circulator and loops back down a return to the hot water heater.

There is still air coming out of the vents, but it is no longer heated.

I have hot water so I know it's not the hot water tank.

The pipe from the hot water heater that feeds the heating coil in the HVAC unit is still warm, but the return back to the hot water heater is cool.

I figure either the pump is broken or the coil in the HVAC unit has become blocked. The pump gets very hot, but seems pretty still. It vibrates a little but not a lot.

I lost the heat pretty suddenly over night one night which leads me to believe the pump stopped working properly.

Any thoughts on whether it is the pump or the coil in the HVAC unit? Any way to check whether it's one or the other?

I would look to the pump or other water control devices. Water coils are very problem free in regards to the water system plugging it up with debris. Make sure the pump is running and all controls are calling for heat.