Preheat error can happen with either with your Nozzle or Heated bed. So 1st make sure with which of these you have the problem.
Things to check:
It is not very common, but there is a chance that extensive printing damaged your heater or thermistor.
The easiest solution is to take a hairdryer and blow hot air at the Hotend or Heatbed. During this process just check if there is any temperature change on the LCD. If there is a change, it means that your thermistor is working, which means that the heater is damaged. If there is no change at all, your thermistor is defective.
Using a multimeter would be the most efficient and accurate way of determining if the components are good or not. It is quite straight forward. See Multimeter usage for how and what to measure.
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