The thermostat does not open if it is stuck or the valve is stuck. In this case, even if the temperature gauge is already in the red zone, the radiator remains cold. Wait until the engine has cooled down, then unscrew the thermostat housing and remove it. You can drive without a thermostat for a while - this will not damage the engine. Operating a vehicle with a defective thermostat can cause severe engine damage. So, the head gasket may burn out, the head of the block may be deformed or even cracked.
The thermostat does not close completely if a foreign body has got in. A symptom of this malfunction is, for example, heating of the radiator immediately after starting a cold engine. The engine warms up very slowly. The thermostat should be replaced as soon as possible, as a malfunctioning thermostat will increase fuel consumption and could damage the engine if it does not reach operating temperature.