There is really not enough information here to go on. If this furnace has a thermocoupling, the pilot will need to be lit manually. Follow the manufacturers lighting instructions carefully. If it still will not light you may have a dirty pilot assembly or a defective gas valve. Modern furnaces do not use a thermocoupling. There is no standing pilot light. Each time the furnace needs to be lit it will be done electronically. Many reasons why it wont light in this case. Safety control could be preventing it from lighting for safety reasons. A bad ignitor, ignition module, circuit board, gas valve, pressure switch, gas supply, induced draft motor, blocked or restricted flue pipe, cracked heat exchanger, dirty flame sensor, ect. ect. ect.
Answers & Comments
There is really not enough information here to go on. If this furnace has a thermocoupling, the pilot will need to be lit manually. Follow the manufacturers lighting instructions carefully. If it still will not light you may have a dirty pilot assembly or a defective gas valve. Modern furnaces do not use a thermocoupling. There is no standing pilot light. Each time the furnace needs to be lit it will be done electronically. Many reasons why it wont light in this case. Safety control could be preventing it from lighting for safety reasons. A bad ignitor, ignition module, circuit board, gas valve, pressure switch, gas supply, induced draft motor, blocked or restricted flue pipe, cracked heat exchanger, dirty flame sensor, ect. ect. ect.