This can be caused by the thermal safety fuse being blown on the hot end. This is the component that is located in the side of the heat sink. One leg of the thermal fuse connects to the heater cartridge and the other connects to the Accelerometer PCB board. When heat is called for by the RAMBo board the red LEDs on the accelerometer board will turn on, showing that heat is being called for but if the thermal fuse is blown the circuit to the heater cartridge is never complete and heat is never attained. Then, when the RAMBo does not see the hot end temperature increase as expected, it defaults to DEF mode since it realizes that there is some sort of a problem.
Figure 1. Thermal Safety Fuse
You should confirm the continuity of the thermal fuse using a multimeter to determine if it has indeed been compromised. Replacement Thermal Safety Fuses
The main question is why has the thermal fuse blown? When you powered on the hot end was the barrel fan (25mm fan blowing across the hot end heat sink) plugged in to the correct port on the RAMBo board and operational? This fan is critical to have running and should be running anytime the machine is powered on. Without it operational you will blow the thermal safety fuse during the heating process requiring it to be changed out.