2004 Mazda 6 cooling system

Check to see if the water pump is pumping. To do this warm up the engine to operating temperature, even if it begins to over heat (a little). Take care and touch the upper radiator hose and lower hose compare temperatures both should be relatively close in temperature.
If one is hot and the other cold (and the thermostat is open) the water pump is no longer pumping coolant resulting in overheating.

Your next probable causes is either restriction (which you should be able to feel on all coolant hoses) or a blown head gasket. Sometimes with blown head gaskets it will push coolant out of the motor and reservoir.

Thanks for the question.