What to do but to tell the congregation God had shut me down for a moment, and this I took to be an indication that He wanted me to shut up. Since no one protested I suggested we recite the Creed together. This certainly restored my wavering self-confidence.
My personal experience tells me that at the very time when I'm doing something important for God, He allows me to have a "power failure" and a "black out" in my preaching. He even allows my inner being to be in a state of flux. He's teaching me never to think in terms of my performance or my personal achievement. Mine is to be the instability of a jelly and the insecurity of walking on shifting sands.