Cooling system heater core went out

You need a live person to look at it, but the common suspects are the water pump, the little bypass hose, and a loose radiator hose clamp. A leaking water pump typically starts out as a very small leak that grows larger over time. If you have a large leak that showed up suddenly, that is more typical of a ruptured hose.

We change antifreeze every two years because the water pump lubricant, corrosion inhibitors, and other additives wear out by that time. If the old coolant is left in there too long, the acids that normally build up in the system do not get neutralized, and that leads to corroded heater cores and radiators. If you suspect that may have caused your heater core problem, the acids may have attacked the water pump too leading to that leaking.

"Core plugs" also corrode through, but when those leak, you will usually find the puddle under one side of the engine, or the coolant will be dripping from between the rear of the engine and the front of the transmission.