At the very begaining of this article, i would like to explain what is room temperature. It is: The thermistor on the tool head will report the temperature of current environment.
1 Won't heat up at current temperature
To this problem, you may set the temperature for the nozzle, and then preheat PLA.
2 Temperature is zero degree and won't heat up
The wire of thermistor is fixed too tight too loose will cause such problem, you can loosen or tighten the screw a little, preheat the PLA again. The loose of heater and the poor connect of thermistor wire can cause such problem too, you may pull and reset the plug of nozzle thermistor and tighten the heater.
3 Drop temperature when heating
In this case, you may test the resistance value of nozzle thermistor, and the normal range is 80-120kohms, if the value is out of this range, the thermistor need to be replaced.
4 Check whether the wires of tool head are plugged tightly.
If all mentioned above don't work, the mainboard must be broken.