Need to know what make, model etc. High or medium efficiency???. If the exhaust blower is not spinning to pressurize the burner chamber it will not allow the gas to turn on.
In the path of the flame on the last burner that starts, there will be a small rod. That rod is the flame sensor. Take a scrub pad and polish it. Also check the electrical connection that goes back to the circuit board. Disconnect any ground wire from the circuit board to furnace cabinet, sand it and replace it.