This is very serious.
Do you have a vet for her?
Has she been spayed?
Has she had vaccinations?
Has she had a check up recently?
What is her diet?
Ferrets commonly develop life-threatening endocrine disease, especially intact females. They can have adrenal problems, reproductive tract problems, and many other diseases that result in weakness, anemia, and a sudden decline.
Nutrition issues can also cause the signs you described.
This is an emergency situation and she needs to see a vet with ferret experience IMMEDIATELY.
check this link to find a vet near you that understands ferrets:
In the meantime, keep her warm, hydrated and try to keep some calories in her. There is not much else you can do at home.