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1. If it has batteries, replace them first.
2.If no batteries, we will move to the furnace and look for a small fuse that may be blown
3.Do you have a volt meter for troubleshooting?
OK did you check and if it has batteries replace them?
OK do you have a gas furnace or electric air handler. Give me model on that unit
OK the air handler should have a little 3 or 5 amp fuse in the red wire coming from transformer that goes to R at control board and thermostat.Locate and check that fuse to see if it is blown.
If you can not find a fuse we will need a meter to troubleshoot.
You can get an inexpensive one at Walmart for about 15 bucks if you would like to troubleshoot and I would be happy to guide you.
Otherwise it is going to be necessary to call in a tech with a meter
There is not a reset as such. You could however have a float switch in your drain that is breaking the power to the thermostat if drain is backed up. Lets look and see if you have a float switch.If so lift it out of drain and see if we get power to thermostat