I am very sorry you lost your best friend Merlin. I do not know what could have happened to your boy, it could be a few things. Sometimes we just never know and when they leave suddenly it leaves us with the question 'Why?'. The only way you would know for sure is a necropsy. We lost one of our boys suddenly at 8.5 yrs old, we don't know why but have suspicions but in the end, it's the pain of sudden loss that hurts so much. Take it easy on yourself and know Merlin is smiling down on you for the love you gave him.