What could be wrong when you start your car and all the coolant runs out

You obviously have a major leak, but if you want a more helpful answer you'll have to tell us where it's leaking from. A leak from the radiator or reservoir would happen without the engine running. A leak from a water pump gasket, bypass hose, head gasket, or freeze plug would leak most when engine is running and coolant is circulating and stop when coolant runs low. If it gushes out until radiator is drained then I would check the freeze plugs.

A blown head gasket might do this too if the exhaust is getting into the cooling system it could cause the coolant to come out of the reservoir overflow.