This means the temperature sensor (the thermistor) is not registering and it may have become unplugged or the connector may not be fully connected. Each kit is pre-tested before shipping to ensure the electronics work, the LCD screen works, the endstops work, the extruder fan/hotend/LED/thermistor all work. Double check the wires in the electronics box (referring to page 5 of the build manual) and make sure the thermistor connector is properly connected.