Head down as under her wing? Or she just sits with her head balanced but low as if she is in a trance?
If the latter, she is most likely simply broody. Silkies are notoriously broody. If broody, she will cluck and fluff and be very unhappy at our presence plus gathering eggs under her if she can find any, otherwise she'll sit on air.
Okay...runny as in watery diarrhea? Then she is sick, possibly coccidiosis, or possibly has worms. You should isolate her from your flock if she has watery diarrhea and is listless and try to figure out what it might be as you may have to treat the whole flock. ANY blood in the diarrhea would indicate coccidiosis and require treatment of the whole flock. Sulmet in the water is a good solution but would need to be applied ASAP. You also would need to disinfect and clean all your waterers and feed dishes and remove and dispose of all wet litter and soggy places in the run.
It is possible it could be something as awful as Mareks as 25 weeks would be a typical time for that to hit, and Silkies are prone to Mareks. Mareks is morphing such that the classic neural symptoms are not always present but rather it presents itself in a bird as listless, slowly failing to thrive until death.
But if it is a broody trance, she will have a really big, once a day, very smelly and thinner poo in a big single blob. The broody hen relieves once a day in magnificent manner.
I'm voting brooding if she has settled in a nest box. If she is simply sitting on the ground somewhere, then I think she may be sick (unless she is low on the pecking order and pushed off of a nest box...then she may be brooding in an odd place...completely possible with a Silkie).
I don't know what you mean by let her out....and I cannot tell you to put a bird down without knowing if the bird is sick or what it could be.
Does she want to sit on eggs? Put an egg in front of her and see if she rolls it under. Then chances are she is brooding.
If she is disinterested and listless in everything, put her in an isolated cage with food and water and observe her. Because of the age and watery diarrhea, I would either worm her or put Sulmet in her water just in case it was coccidiosis. You could also put some antibiotics in her water to see if it could be bacterial. I try to give a bird at least a few days to a week to determine if she can rally on her own or with some care before contemplating putting her down....UNLESS it is very obvious it is a very sick bird with something very contagious...then I will put a bird down to protect my flock.
If it is Mareks, keep her isolated (although the flock is already exposed), and observe. She will be disinterested in life, and if at the end of a week she seems to remain disinterested in spite of Sulmet and/or antibiotics...not eating or drinking...then yes, you may need to put her down. I recommend the CO2 method by using dry ice in a 5 gallon bucket...it is quick and painless IF you build a nice cloud first (1/4 lb dry ice with some warm water at bottom of bucket, towel or styrofoam on top of that) then put the bird into the cloud and close the lid on the bucket. Unconsciousness occurs within seconds, death within a minute.
...also a broody hen will still be in a trance if you pick her up and put her on the ground...she will then awaken and move around stiffly for a few minutes to go back into her nest. Some run around like crazy birds squawking for a few minutes, peck around, eat a little, drink a little, leave one huge poo, then go back to the nest.