I'm fairly new to keeping chickens, so might be talking rubbish, but my thoughts are-
1) Is she laying normally ?
Maybe she's eggbound and trying to get the egg out.
2) Is she up to date with worming and mite treatment ?
3) Since it was a nice day yesterday (well, here anyway) maybe she's got too much sun ?
4) Does she seem ok in herself, or does she look stressed ?
If she's not eating or drinking it doesn't sound good - but that may just be a temporary thing, try offering her favourite food and see if she's tempted.
5) Are the other chickens avoiding her or harrassing her ? If they're giving her a hard time, I'd be tempted to move her out (if you can) and just keep her seperate til you see if there's any change.
Hopefully someone else more experienced will be along soon, good luck and let us know how she gets on.
Karen x