First of all, don't panic!! I have spent many hours in our goat sheds watching a goat that does not look her normal self, believe me
Has this goat been out on grass during today? Does her sides look bloated, and feel hard? If so, it could be just a matter of getting her to belch!! Can you hear her tummy rumbling if you put your head close to her side? Bicarb of soda given on a spoon on her tongue, or put some in warm water and give as a drink, then massage her sides, and try to walk her round, it usually does the trick, and they burp and start eating.
If she looks fluffed up, it could be she has caught a chill. A dog coat is good, but failing that a blanket, and plenty of bedding round her. If it is a chill, be prepared for her to scour as well, and have a runny nose. A sick goat looks very sorry for itself.
Quite often, if they are under the weather, they refuse feed, but will eat hay, so offer her some hay and see if she has that. Willow leaves are usually liked in situations like this, mine strip the bark off.
Finally, you are sure she has not eaten anything poisonous while outside? Or pigged out on feed?
If she looks really ill, or you are unsure, then I would call the vet in the morning, even just for reassurance over the phone.
Oh, and do you think she is coming into season? I have a couple of goats who go off their food a day or so before they start bleating and tail wagging!!