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All air conditioners require the air filter to be clear of dirt buildup. Check the air filter on your furnace to make sure that it is clean since it your furnace that moves the air across the air conditioning coil and gives you your cold air. If your air filter is clear of all dirt then you most likely have a low charge on your air conditioner and require a service technician.
Most likely your thermostat is digital. All digital thermostats require a battery change roughly once a year. Your thermostat may even have a low battery indicator notifying you that the battery is low. If you have changed the batteries and furnace is still not running you may need a service call.