Two things. Firstly, I go to sleep, because generally block hits me after too many hours at the keys. My dreams are often infiltrated by my characters, and my subconscious has been know to solve their complicated riddles. If I still wake up struggling against the writer's wall, I start a new story. I explore new characters and outline new books. I have lots of WIPs created that way. A little time away from the problem in a novel often allows my brain to process the issues, unclogs the pipes, so that when I come back to it the story flows again.