You can help by being there for them.
For a number of reasons they may not be ready or comfortable sharing their problems. Forcing the issue and demanding they tell you what is wrong will add to the pressure they feel they are under and will more than likely make them go further inwards.
I feel the best approach is to show them you are always available to talk to if they need or want to, some people need some distance to put things into perspective before opening up to other people.
Also I find each time you recount a bad experience you are subjected to the same emotions again. It is not always a good idea to tell everyone your problems; the negative emotions may be intensified.