Hi Sarah, sorry to hear about your sneezing chivken. There are several reasons why this might be happening, and a few more details would be helpful.
Is she a newcomer to your flock? If so, she might have brought in a respiratory illness, perhaps triggered by the stress of moving home. This is quite common in birds that have just changed to a strange environment, which us why its a good idea to isolate them for at least a couple of weeks on arrival, as a quarantine measure if possible.
How many chickens do you have, and is this one the only sneezer? If so, its probably something restricted to her, for the time being at least,
rather than an environmental cause such as dusty bedding which would affect all of them.
Has she done this ever since you got her, or is it more recent? If so, when did it start?
What sort of bedding do you use in the coop and nestboxes? If its at all dusty, its likely to affect the birds' breathing and make them sneeze, just as it would for most people if they had to sleep in there. Hay and sawdust are best avoided, for this reason.
What's the ventilation like in your coop? Hens are best given full ventilation even in cold weather, as too little will often produce respiratory complaints from breathing stale air and condensation within the poorly ventilated coop. If the coop is dry and sheltered from direct howling winds or draughts, you don't need to worry about keeping the girls warm. Many people understandably worry about whether their birds are cold at night, but forget how well insulated they are under those brilliant feather coats.
Is there any discharge from her eyes or nose? If you pick her up and put your ear to her chest, can you hear any wheezing or rattling sounds?
Is she eating, drinking and pooing normally, so far as you can tell? Does she appear perky and involved in normal activity, and is she laying?
What breed is she, and how old?
If you can tell us a bit more, I'm sure somebody will be able to come up with ideas to help.