That's always hard. There's almost nothing to do. You don't want to make the pianist look bad because then you look bad to the auditioners. Just take comfort in the fact that it happens to everyone. You almost NEVER want to ask to start over, though.
"This table, he is over one hundred years old. If I could, I would take an old gramophone needle and run it along the surface of the wood. To hear the music of the voices. All that was said." - Doug Wright, I Am My Own Wife