It sounds like they are in a major depressive episode which can be dangerous. First, they do need to know that you're there for them. They won't be as responsive as normal because they simply can't respond, but it helps knowing someone is there. Text messages, a card and such are all you really can do unless they come to you.

Now, if they completely stop responding, then I agree with Ganganthefatman in that you need to visit just to check up on their safety.