chick won't stand up! help!

hello chicken peeps,

i have a 3.5 week old ameraucana chick who was strong and fine yesterday, but today she won't extend her legs and stand up or walk. we had just transferred the 2 chicks yesterday to a closed-off section of the coop, so that they could see the older 2 chickens and have a bigger space, but the silkies can't actually be in the same space with them and there was no evidence of predators or 'fowl' play (our coop is quite safe!). but when i went to let out the big girls earlier and check the chicks, she seemed like she was trying to stand up but could not. i picked her up and she's in a box in the house now, food and water within neck reaching distance.

she is eating and drinking, but also seems a bit lethargic. when i hold her and extend each leg, they don't seem actively painful (she doesn't jerk away or tense up when i touch any part of either leg). she will curl her toes around my finger, but won't stand up. her legs are not misshapen, and there is not an obvious wound, redness or heat coming from anywhere. i can't think of how she would've pulled, bent or strained her legs due to a dangerous aspect of her coop section, but it's possible i guess. i am a very new chicken owner and have no experience with chick medicine!

anyway, any feedback would be awesome. i would really love to hear that sometimes chicks pull a knee or something and it's fine in a few days! eh? eh? thanks for helping me out, oh chicken folk!

--cameroneil