Great question! This can mean a couple of things. It could be that Lancelot is sick. He could have diarrhea, stomach upset, may be in pain, have feline idiopathic cystitis, or bladder stones. He could be feeling so badly that it's not worth him getting out of the box only to have to return again. There are other reasons, such as feeling territorial and wanting to keep other cats from using it, but it's very important to rule out a medical cause first. He should see a vet for a check up, and I do recommend getting blood work and a urinalysis done. If he does have a medical reason for this behavior, the vet can diagnose it and discuss treatment options If he's healthy, the vet can discuss behavioral causes and treatment for that. I'd take him into the vet in the morning. I hope this helps!