If your cat has suddenly taken to hiding in a closet rather than spend time with her family, then the first thing to do is take her to the vet. This will rule out any physical reason for her wanting to hide; cats in the wild who are feeling poorly prefer to hide because it gives them a chance to rest and heal without much interruption. You didn't mention whether or not she was still eating and using her litter box. If you have noticed a decrease in either activity, a trip to the vet is very necessary. She might have developed a URI or UTI, she could have nibbled on a plant she shouldn't have, she might have gotten into the trash and had something disagree with her, or she could have decided to munch on something she should have - like a string or twist-tie - and it could be causing a bowel blockage.

Before looking for other causes, start there. Once physical problems are ruled out, then you can begin to tackle psychological problems (a much more difficult task, to be sure).

Things to consider:
Have you recently had a guest who might have upset your cat, or have you had workers/contractors in your home lately? Sometimes strangers can upset an insecure cat, or a cat can take such a dislike to someone for an unknown reason that the cat finds it difficult to recover. I always keep notes on people my cats don't like (they always know best).

Can your cat be engaged in play and does she still play as before? If she likes catnip, does she still like it and react the same? My cats have a brand loyalty - my homegrown or one special brand - and won't react to any others.

Anyway, if it's not physical, your battle is about 1/4 over. Good Luck!