One of the hardest things in writing is working out what is going to happen next. There are a few exercises you can try to make it easier. Sometimes, when I get stuck, I grab a handful of 5″ x 7″ cards and start by writing an opening plot line on one of them. Next, I take two and write one of two possibilities on each. For each of those, I add another two possible outcomes and so forth, until I have the makings of several different story paths. You can either find a blank wall to post these on and track the possibilities or use a big stretch of empty carpet.
After a while, you can trace the outline of a new plot. From there, you can start to flesh out the story. You may not decide to use any of them, but they can provide inspiration for something new or at the very least, alleviate writers block.
So, grab some cards and get going.