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Hi I'm Brian and I'm here to help!
This problem is typically do to 3 successive high temperature limit trips in which case turning power off to the furnace for one minute then restoring it should reset it. OR, the flame has rolled out of the burners and the roll-out switch has tripped in which case the roll-out switch must be found near the burner nozzles and be reset by pushing in the tiny little button in the center of it.
The high temp limit trip is often caused by a dirty or plugged air filter so you should check that.
The roll-out switch trip can be caused by quite a few things including a gust of air that blew air down the flue among many other reasons. If you can get the unit reset and it continues to happen, then it's best to have a technician look at the furnace to determine the cause.
If you can give me the model number of the furnace, I may be able to shed more light on the subject.