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If you reset the power to the furnace and only the fan comes on and it does not try to go through it's ignition sequence.
Then you have a limit tripped. There should be a blinking LED on your furnace control board giving you a failure diagnostic blink code.
There should be a few limits that look like this that you can reset by pressing in this small reset button.
If those do not reset the furnace then the main limit is tripped. This limit should reset once power is reset so if it isn't it is stuck. You can jump across these two wires to test and see if this is the limit that is tripped.