I have never told my wife that I had a vasectomy?

I had the procedure done almost 4 years ago, and I never told my wife about it. Yesterday, she came home from the doctor, and informed me that she is pregnant. I know that there is no possible way that the baby could be mine. I was just in shock, and I had no idea what to say or do. I still have not said anything to her about it. What should I do or say when I go home from work this afternoon?