Do you have an idea of what needs to happen next? If you don't, it is a good idea to just think about it and write it down in a simple way to give yourself a road map and connect the dots.
If it's the writing itself that gives you trouble, it's just a matter of patience. The right words will eventually come. Sometimes it will not be perfect though, so it's good to leave it for a few days, then come back and retouch it.