It sounds as if there's been a lot of stress in your daughter's life recently - and in yours as well!
First of all, I wouldn't worry much about the hiding. It's probably a sign that your daughter is feeling overwhelmed. An illness can disrupt a child's sleep pattern, as can starting at a new child care center.
Children this age are also extremely sensitive to their parents' emotions. If you're feeling frazzled, she'll sense your tension and become more anxious herself. This makes the cycle grow worse from day to day.
Probably the best thing you can do is spend some extra time with your daughter each day in calm, soothing situations. Have her take a bath as part of her bedtime ritual, perhaps. Spend time cuddling with her as you read her a story. Stroke her hair if she finds that calming.
Once she feels more relaxed around you, she'll have less of a need to hide in the closet.