That's really something you have to find out for yourself, but I'll try anyways. I don't think I've ever really been sad, but let's see if I can't think of something...do you know why you've been blue? Maybe if you get at the heart of the matter you'll be able to 'fix' it?
But joy's everywhere, you just need to know how to see it in a person's laugh, in a child's enthusiasm, in the beauty of nature, in comic strips, and books. In movies, and songs, and in the people you love. It's in the very air, but so too is grief. But even in grief there is joy in knowledge that no night goes on forever, there will always be a breaking dawn. So maybe this will just make you stronger, let you see the joy more. Who knows but you?