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What is model on the thermostat?
Also try pressing the HOLD button and raising to 70 and tell me if it stays are reverts back to 40.
If thermostat will not stay at set temp with hold displayed it may have taken a power spike and have to be replaced.
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.