First check the resistance of the thermistor as is, i.e. leave it on the hot end. If it reads lower than 100 ohms it's broken, if it reads 0 it's shorted, check the leads making contact on the heater block. If it reads much higher than 100k at ambient like above 1 mega ohm.
Now check at which connector it's connected to. Maybe you've connected it to the 2nd extruder connector. Also lookup the connector layout of your board to make sure you're using the correct one.
Check the board you're using in firmware and pay attention to what combination of extruders, bed and fan you have configured.
If it still isn't working check what pin is assigned to the connector using the boards schematic. Look in pins.h if the pin assignment matches.
You can try updating the firmware.