If you suspect that this hen is malnourished, it could be because she is prevented by the others from getting to the feeder. If this is the case, you'll need to address that issue. You can refer to this
https://www.backyardchickens.com/articles/chicken-bully-chicken-victim-a-two-sided-issue.73923/ if you determine she's being bullied away from the feeder and not just broody.
The fact she was all muddy gives me the idea she could have been the victim of an attack where several chickens stand on the victim and hammer her into the mud with their beaks. When that happens to a chicken, they normally continue to lie there and not move out of fear that it will happen again and also because they have resigned themselves to the role of being a victim. A chicken that is bullied to this extent will feel it's what they deserve and they have no right to eat. You need to change that or she could die.