There could be multiple reasons for the issue, but the most common issue is the temperature sensor. We can test it:
Disconnect the hot end from the control board and place the sensor on a warm plate and after some time place a candle closer to your sensor, with a 5 mm gap. If your board is able to display the variable temperature, then we can say the temperature sensor is fine.
Or if you're good with electronics and programming, use Arduino/ATtiny/Arm controller to measure temperature using the temp sensor of your 3D printer.
The next issue could be some component in your control board, it happened with my printer, in my case, resistor and two capacitors were at fault.