Wow...this does happen, and when it does, it's time to take stock in the rest of the relationship.
If your relationship was all about sex, it's over, and good riddance to it.
If you love him for other reasons, and he loves you, than you may have something salvagable and worth saving.
Yes, he could be depressed, or cheating, or both...you need to know. Have a heart-to-heart, but try not to sound accusing; try to sound desperate for him and wanting to know what you can do to make things better (trust me, this will get you further). Either way, you'll have an idea of what you have on your hands, and what it's worth.
If it falls apart then, call me. I love sex.

j/k