First point of call replace the thermostat.
Next does the fan kick in?, if it doesn't replace the fan sensor switch in the thermostat housing, check the fan itself actually works, do that by taking the plug off the sensor on the thermostat while the engine is running, if the fan motor is ok it will run continuously.
Last I can think of. if above does not work, is the impellor in the water pump has died, that will need new water pump.

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