Best left to a professional but you might be able to fix simple things. The pilot light should have a small metal rod or two beside it with the flame heating them up. This is a safety system designed to keep the fireplace from lighting if the pilot light is extinguished. If the rod is too cold the fireplace won’t turn on the gas valve to light. To test where the problem might lie, use a long-nosed butane grill lighter to add additional heat to those metal rods to see if the fireplace will light. Be ready to step back if it lights the fireplace to avoid hairless arms. You may need to hold the flame of the grill lighter on the metal rod for 30-45 seconds (with the switch turned on) to heat it up.
If the fireplace starts then it’s likely 1. the pilot light needs to be turned up a little. check the gas valve on the internet to see how and/or 2. the thin tube feeding gas to the pilot may need cleaning . they commonly get spider webs inside them (if you remove it turn OFF THE GAS and don’t kink the tube reinstalling it.