My husband and I went through something very similar a couple weeks ago- someone suggested the chick might be dehydrated or vitamin deficient and said that administering a vitamin called poly-vi-sol without iron could help. It's a supplement for human babies that can be found at most drug...
Hello (this is Todd510's wife),
Thank you for the feedback! Several days of vitamin/electrolyte-enhanced water and supplementing with poly-vi-sol (without iron) did the trick! Our chick gradually became more active and eating/drinking more regularly; today she is just about at 100%...