When a vehicles thermostat is stuck in the closed position it won't allow the coolant to circulate properly throughout your vehicles engine and through the radiator like it should.
If the coolant can't circulate through the vehicles radiator and back through the engine like it should then the engine could overheat because it won't be able to cool down properly.
A stuck radiator thermostat can also cause no heat to come out of your vehicles heater vents in the winter.
So if your vehicle is not putting out heat through the heater vents then your thermostat could likely be stuck closed and need replaced or your heater core could be bad.