My cat started suddenly having this problem. Our solution was to get him a new litter box that was much bigger than his old one and fill it with the same litter we were using. He suddenly was interested in it again and he hasn't had any issues since.
I'm definitely planning on getting a new litter box for him. I just haven't gotten to it yet... I'm probably going to go tomorrow because I don't have work and a quick run to the pet store isn't going to interfere with getting my homework done. When I go, I'll look into those products if they have them.