See, if my friend used to be a cheerful and talkative person but all of a sudden, now has started to be all quiet and gloomy, then I would simply presume that there, for sure, are some problems that he is going through. They could be from his family side, academic problems or simply the social issues. I will therefore try my best to console him/her and will try to find out what's the reason behind his/her sorrow. Definitely as a friend, he/she will disclose her issues and hopefully I'll be able to help him/her out.
Hope that's of some help. :)