We turned off the furnace during the summer, now it will not fire, it acts as though it might but will not, oh abd the gas is on

Depending on where your shutoff valve is located, you may have air in the system. Does this model have a pilot or other ignition source?

What is the make and model of the furnace. How old is it. The more info you can give the better we can try to help. Tell us exactly what is taking place as soon as you turn the thermostat up.