Oh, by the way... this is a Silkie, Bantam? How old? How cold is it getting? Maybe put a thermometer out in the "coop". How is the rest of the flock? As long as they are dry, protected from the wind, etc... Chickens have a down blanket on -- feather jacket. There are people here that have "well insulated" coops and only turn on the heater when it gets to - 20, that's 20 below 0.
If it's not insulated well, not warm enough ... how about getting a good thick waterproof tarp to throw over it, and tie it down with tent stakes? (Make sure it can't get puddles of rain on it.. that it's sloaped so as to not "hold" rain or snow.)
Could there be something else going on? Can you answer all the other questions ... like: what kind of "nesting" material do you use? How old is the flock? How old is the "sick" one? Have you wormed? What do you feed them? Do they free range? Clean coop, roosts, run -- fresh clean water and food? Could they / she have gotten into moldy food or something toxic? Everyone else doing ok with the weather? How about a heat lamp? Can you, is it safe to put a brooder light with a heat lamp (bulb) in it into the coop, it would have to stay dry and have electricity available.
Do keep a sick and weak bird separate and isolated from the rest -- until they are well and strong, and until you find out what is going on with her. Is she the only Silkie Bantam you have?
I'm sorry she is sickly... best of luck... you might let her out, put her with the flock during the day if she is well. And, if it's warm enough.
Take care...