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Hello and thanks for using this service, if you have been waiting do not worry or get upset I am just one of the techs on here and I try to get to as many customers as I can and I just picked up your question now. Please give me time to respond sometimes I get your replies late so there may be a delay in my response.
That means that either you have lost the 24 volt supply or you do not have a wire connected to the C terminal on the unit. If you do not have a wire connected to the C terminal you would then need to charge the thermostat every so many days or that will happen. If you do have a wire connected to the C terminal at the thermostat, go to the furnace and get to the main board in the furnace. There should be a 3amp SLOT FUSE on the board in the furnace, if that is burned out that will cause that issue,
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The more realistic issue is that the furnace has run on limit or tripped a roll out switch. Limit is caused by lack of air flow, dirty filter, bad blower etc.
Constant fan (cool air) with no communication to thermostat is 99% tripped furnace safety. If you choose to move forward to check a few things I can help.