Shift over and write something else for a few hours--or if it's really bad, a few days. The Kiernan novella, Time's Echo, was partially written by the time I finished Time's Edge, the second book in the series, because I'd wander over and play around in his mind for a few hours if I got stuck on some aspect of Time's Edge. I'm trying to do the same this time with the second novella, Time's Mirror, which will be from Prudence's point of view. I find that writing something else is a bit like stretching before exercise. It loosens up the brain and gets the creative energy flowing. Usually that means that I can work my way through the problem spot when I come back to the main project that I'm writing.