Brand,model & year of furnace always helps. Older gas furnace means different things to different folks.
Are you talking about a straight pilot light with a thermocouple going out within 45 seconds? Are you talking about an ignition system with a flame sensor locking out?
If a gas valve that has a continuous 24V shuts down then either the gas valve is faulty, the gas supply is inadequate, or a primary safety control that is connected to the gas valve has shut it down.