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What is model on thermostat?
Have you tried removing batteries if applicable for a few seconds and replacing?
Second if removing and replacing batteries dos not help. using a straightened paper clip see if it will reset by sticking in the little hole on front.
OK at this point it appears thermostat has taken a power spike.We have tried all of the tricks to reset based on manufacturer's suggestions.At this point, since thermostat will not respond it will be necessary to replace, but before you do, there is one trick I have seen work to reset these digital thermostats that works sometimes. That is to remove batteries and install them backwards for 5 seconds, Then remove and install properly,If this does not reset the little computer board in the thermostat you will need to replace the thermostat.
Note: Occasionally , this will not work and will even cause thermostat to start smoking.If so simply remove batteries.If thermostat is not working already, it is worthless anyway, so no harm done .I have seen it work about 20 per cent of the time. So if that is something you are comfortable doing would try it.Could save you having to buy another thermostat.Otherwise, just go ahead and replace it.
If none of that works you will need to replace thermostat.Let me know if you would like recommendations.I would stay away from Rittemp as they are pretty much junk and highly susceptable to issues like this.