Mine has been doing the same thing for 15 years. Every Thanksgiving and Christmas with a house full of people and NO HEAT. Blower runs, igniter clicks away, but the gas valve ain't turning on. Flip off the power for couple minutes, back on and viola, heat! When it decides that's it, we repeat the process. Sometimes days, sometimes hours, sometimes weeks. I read awhile back it is the control board. It has a cracked or broken land and when it is cold it contracts and stops allowing the electricity to flow to the right circuit. Board is $500 and NO GUARANTEE!
Noticed no one answered you guys so I thought I would at least give you some info.