If you are comfortable doing it I would check the thermo-couple, However here is one more thought...
Has the fireplace been hit or knocked with anything heavy like furniture ?
There is a limit switch on the outside of most units that is pressed between the fireplace and the wall / bracing holding the fireplace. If the fireplace is hit hard enough to move it, the limit switch will open. This opens the circuit to the coil on the gas valve and will not let it close electronically, only manually, which is what you are doing when you try to start it. This limit switch is a safety switch incase someone drives into the side of your house.. it shuts off the gas immediately to turn the fire place off….
This happened to my fireplace about 3 years ago and it took me almost a day to find the darn switch. There is probably a jumper on the front panel that you can short out to test this switch, however you would probably need the schematics for your fireplace to find it.
If you are not comfortable doing any of this I would call in a professional.
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