Sometimes depression can manifest as a lack of emotion of any sort--a flat, dull, tired feeling that takes away your enjoyment of life. That's how it was for me, and how it is for many people who get clinical depression; there's just not enough energy left to feel really sad. It's more like you've started editing your perceptions of everything so that it seems dull, boring, hopeless, and ultimately not worth bothering with.
That said, I have very strong emotions normally, both positive and negative. Part of my particular sort of autism actually involves not being able to inhibit emotional expression very well, so that if I'm feeling anything but calm, whether that's interested, excited, sad, angry, or anything else, it's obvious to anyone around me because I don't know how to keep feelings secret.