No water pressure in heating circuit
It sounds like you fill valve is not properly set, or has failed. Do not run the furnace until you are sure the system (boiler shell, circulating pipes, and radiators) are completely filled with water.

If the water level in the boiler gets too low the system could be damaged. Boilers usually have a fuse plug that limits the damage if the water level fails, but that plug cannot be reset and is expensive to replace.

A typical circulating hot-water heating system has an automatic fill valve connected to the house water supply, which admits water to the system until the pressure rises to a preset level. (Usually the level is set to match the total height of the system.) This valve can become blocked by a tiny amount of shmutz in the supply line.

Sometimes you can clean and reset the automatic fill valve but sometimes you have to replace it.