Sounds like a rocker arm problem. You can easily check this by pulling the valve cover and looking at all 8 of them in the top of the head to see which one is the issue. It will either be broken or fallen off to the side of the Valve spring. Depending on you mechanical experience you may want to get some one experienced to assist you. You should invest in a set of rocker arm stoppers when you are in there and have them installed. This will help to avoid any further issues.