If your bed reads below the minimum temp, it won't heat. Same goes for the hot end (for hot end readings). This is because a very "low" reading is also the same as having the thermistor disconnected. If the thermistor is disconnected, you can't tell when to switch off, which would be dangerous.
1. Try heating the bed to ~20 deg C before turning it on. A hair dryer or even just putting your hand on the bed might help. If this works, then you want to change the minimum temp reading in Marlin for the bed.
2. Got the right thermistor settings/calibration? Haven't accidentally electrically shorted the heated bed to the thermistor somehow? Got the bed on the right connector (eg: not the one for the second extruder) ??
1. Try heating the bed to ~20 deg C before turning it on. A hair dryer or even just putting your hand on the bed might help. If this works, then you want to change the minimum temp reading in Marlin for the bed.
2. Got the right thermistor settings/calibration? Haven't accidentally electrically shorted the heated bed to the thermistor somehow? Got the bed on the right connector (eg: not the one for the second extruder) ??