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It is likely that the thermostat is bad, or that there is no power to the control circuit because the air handler or furnace is unplugged, or has tripped its breaker...... or there are problems internal to the air handler that have interrupted the 24volt control power.
Tell me a lot about your technical background and if you have a multi meter or not and I can coach you extensively from there.
We are on the honor system here. Please remember to rate my service before you leave today, the company only pays me if you rate my work. It is how I earn a living, and does not increase your charges.
If you need anything else, just let me know. You can continue asking follow-up questions at no additional cost.