Fireplace wont come on

Try tapping on it a few times. Sometimes they stick. Also check the thermopile. It might not be putting out enough power to power the gas valve. The battery only powers the thermostat- a weak thermopile may keep the pilot on but not be able to pull the gas valve open. Sometimes just cleaning the flame sensor and contacts will get it working again (particularly on units that have sat unused for several seasons) If it is a "milivolt" system - and that means the unit will run even without power.

Looking at the part number you give it's an electronic ignition unit - so no thermopile - runs off line power. Still try tapping the gas valve as you turn it on and off - but you will LIKELY need a new gas valve.

Check the thermocouple that's in the pilot light flame. It has to produce enough voltage to power and open the gas valve. It's there so that if no pilot flame is there, the gas valve won't open and if the flame goes out after it's open, it will close. If you have a meter I'm sure there are plenty of videos online. They are cheap and easy to replace if that's the problem, which is very likely. If the pilot is lit, manually turning the gas valve to on doesn't light it, it's either the thermocouple, the wiring between it and the gas valve, or the gas valve.