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As a reliable friend, you may want to let her know that you're there for her. Unless she is refusing to talk to you for some reason, then yes, try to keep in touch with her. A friend in need would benefit from emotional support even if you do not know the cause for her shutting down. She herself may not know what she's experiencing; depressed mood may lead to social withdrawal.
If you detach yourself from her at this time, she may believe that she's not worthy to be around when she feels down. You can still behave the same way as you've done when she felt better or was more engaged with you. It is possible that through your contact, she cane come out of her shell faster than if you leave her alone. You are not intrusive by trying to be supportive.