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Hello, locate the fire/smoke detector terminals on the units printed circuit card and fit a jumper wire across them....if the unit runs you know that a fire or smoke detector has tripped or a wire has broken or come loose in that circuit.
If the system does not resume normal operations after you have the jumper in place, and cycle the power OFF then back ON to the unit, then the problem would most likely be in one of the other safety circuits, such as high pressure or low pressure cut on the refrigeration / heat pump circuit.
I am off to bed shortly, If I miss you tonight I will pick your response up first thing in the morning,