Gas fireplace will not come on or it goes off minutes after coming on
From the sound of it some kind of safety has kicked in. There's probably a thermocouple that's connected to a cut off valve of some sort. If the gas is on, but the thermocouple is too cold, it cuts off the gas (this is intended to prevent something blowing out the flame from filling your home with gas!).

Look around and see if you can spot the thermocouple and make sure it's not bent out of the way or something silly like that. Beyond that, I'd call for service on the thing.