I'm been fortunate in that I haven't experienced this. If I'm struggling I realize there is something I don't know. I love to research. I want to know my characters so maybe they haven't introduced themselves in such a way that I'm inspired to proceed. The answer for me is to do more background work. To find people who have experienced what I'm writing about and talk with them. As in the case of Andrea in Wood's End I talked to people who had experienced abusive cults. For me... if there is a lapse in productivity I realize that maybe I lack empathy for my characters in a way that's missing.