I got a Goodman GMH950904cx furnace. I installed it about a month ago, and it worked fine when I turned it on. Recently, the weather chilled and I tried to turn on the furnace everything worked except there is no gas coming up.

The motor runs, and ignitor light up but no GAS. The gas tank is almost full and the gas stove works except the furnace.

Please give me some idea of what might cause the problem?

Are you saying ,the fan will run when stat is set on fan only,? this tell me you have power,so first check and make sure the on/off switch on the gas valve is in on position, then make sure any cut-off valves in gas line are open to furnace