How to help someone who is shutting me out?

Aw. From what you've described your friend is really struggling and it's lovely you wish to help her. One of the best ways might just be to give her some space but let you know you are always there for her. A common symptom of depression is social isolation and withdrawal from activities and friendships the sufferer used to enjoy. I doubt she's pulled back from your friendship or resisted your attempts to reach out to her and reconnect due to anything that you have done or dislike for you. Depression can really zap a person and dim their energy and lightness. My friend who has Major Depressive Disorder described it as being like having invisible weights dragging you down. She lost so much of her joy. She used to be an extrovert who gained energy from the company of others but she became an introvert who felt drained in social settings. It could be that your friend is this way, too, and that she doesn't have the stamina to maintain a social life right now.

I would just invite her to do chilled out things now and then like grab coffee or something along those lines. Perhaps pop a card in the mail for her every so often just to say hello and that you're thinking of her. I know it seems really old fashioned, but that's sort of why it's lovely. It's unexpected and it's a very mellow way of remaining in her life and still showing her that you care without her feeling like you have expectations for her that she can't meet right now. Hopefully she'll get help for her depression and will return to the friendship.