If I find myself in this situation, I have several methods of dealing with this potentially frustrating event. The first thing I do, before everything else, is not punish myself, and just accept that my mind needs refreshing from time to time. Sometimes I leave the WIP and concentrate on another one to change my mental scenery. However, I may use the mind-map method to plan the plot and sub-plots, allowing a helicopter-mind view to see the bigger picture. At other times, I just write, letting the characters have their say in what happens, and sometimes I'm really surprised with what comes out; although not always successful, I might add! I also find simple things like walking the dog, reading, or watching TV can be enough to spark a flame. The trick is here, to always have a notebook to hand to jot down those thoughts.