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If your reservoir bottle is full the coolant escaped the pressurized cooling system. This means you either overheated (overexpansion of coolant escaped past pressure cap as designed), the pressure cap failed (bleeding coolant off at a lower pressure allowing it to expand more), or your head gasket failed and combustion gases/exhaust began displacing coolant from inside the engine.
Easiest to check is radiator cap as they're usually a cheap replacement. If you see an oily sheen or skim on your coolant when removing the radiator cap you may have had a head gasket leak. I think there is a way to test for the presence of exhaust gases in the coolant but I don't know the best way to actually do this. If you don't have reason to suspect a bad head gasket and the radiator cap appears to work something is causing you to overheat. An air leak in a hose could cause an air bubble to become trapped in the system which will have to be burped out. The thermostat could be failing to open all the way. A cavitation-eroded water pump impeller can cause overheating, as can a scaled-up radiator or bad fan clutch.
Start with the simpler/cheaper/easier checks and proceed from there.
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