It depends on how close the deadline is! Actually I find Nora Roberts's advice to be excellent -- sometimes you just have to put your hands on the keyboard and trust the *girls in the basement* to show up. If truly blocked, it can be because I took a wrong turning. If it is because there is intense emotional turmoil in other parts of my life, this can be a signal to refill the well and do lots of unrelated reading. But because writing is a bit like breathing for me, putting my hands on the keyboard can work.