|If the heat sink is installed right then you may not have applied the thermal compound correctly.|
73 degrees C is too hot for just sitting in the BIOS screens.
Did you change any settings in the BIOS? Overclocking or applying the wrong voltage to the processor can cause high temps.
Heat sink fan that is running too slow can also cause it. Is the heat sink and fan free of dust/dirt? Don't use a vacuum cleaner. Blow out with canned or compressed air as needed.
Did you say this started all of a sudden? Were you inside the case just prior to that? Some heat sinks rely on plastic clips to hold the heat sink in place. They can fail. If you can push in on the heat sink and it moves that may be your problem.