Ford's spark plugs on 4.6 and 5.4 engines from 1999 to 2003 have a design issue that make the spark plugs prone to blowing out. There are only four aluminum threads that hold the plugs in. The Ford dealers will want to replace the heads rather than repair them. Thin repair inserts may blow out again, so using a thick proven repair insert is recommended like the Calvan 389-100. Speed shops use them when modifying and rebuilding Ford 4.6 and 5.4 engines, the repair lasts the life of the engine. It could be a faulty plug or a vacuum leak that causes the cylinder to run far too lean. Replace a couple plugs, run it for a few miles (20 to 50) then pull the plugs and see what color they are. If the plug in the cylinder that failed is too bright, you're probably running lean from a vacuum leak. Unless you mean the plug simply came out of the head. If so, maybe it just got left loose and eventually worked it's way out. Also possible the threads are stripped out of the spark plug hole and the plug actually blew out. Take a look at the threads and see.