Pull your display off wall and see if your unit has batteries. If you have a blue wire or, a wire connected to the "B" terminal in your thermostat(the part connected to wall, this should produce the power to show display. If you do not have a blue wire, it is dependant on the batteries for display. Change out batteries and see what happens. Obviously with no display, your system won't work. If this doesn't solve your problem, replace the thermostat.
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Pull your display off wall and see if your unit has batteries. If you have a blue wire or, a wire connected to the "B" terminal in your thermostat(the part connected to wall, this should produce the power to show display. If you do not have a blue wire, it is dependant on the batteries for display. Change out batteries and see what happens. Obviously with no display, your system won't work. If this doesn't solve your problem, replace the thermostat.