How to deal with stress/depression 10 POINTS?

My 16 year-old sister is kinda moody a few days ago. I asked her, she said that she was just 'tired' until I realize that she woke up at 12.00 a.m. and cried. She has worked the whole day on her school holidays homework, but she said she felt like she has not done anything. What can I do to stop this? Please help! I am not an expert in psychology... If this goes on, my sister would have gone INSANE! Please be a helping hand and at least give me a few tips on how to deal/prevent this depression from her. Is it just hormones? I can't let it be... Swing to moody... D:! Her sour face 'magically' 'poofed' out on her face. Please answer. :) For me, his twelve-year-old brother..... :)