The most common reasons for gas fireplaces not lighting are pilot lights, clogged tubing and worn out thermocouples. While only experts should fix broken gas fireplaces, the average layman can identify the problem fairly easy.
Check the main pilot light and see if there is a tiny flame. Pilot lights can be blown out by wind coming down the chimney or mechanical faults. Look at the manufacturer's manual to see how to restart the pilot light. Most fireplaces have a pilot light button for easy use.
If the pilot light has been out for a long amount of time, the tubing may need to be cleared of any excess buildup. If the fireplace has a pilot light button, hold it down for several minutes to let air clear through the tubing. If there is no pilot light button, find some compressed air and blow in the area of the pilot light to clear any debris.
If turning the pilot light on, clearing the tubing and clearing the area with compressed air provides no help, there is most likely a worn out thermocouple. At this point, the best way to resolve the issue is to contact a local gas fireplace repair service.