Let your imagination run free with ideas. Don't think of ways to complete the story but continue it. For each idea, ask yourself "what if" and then continue the story.
For example, let's begin our story with two boys playing hooky from school. By using the "what if" process, we can continue the story by asking "what if these two boys went to the circus instead of school?"
From the "circus" lead, we can extend the "what if" question to encompass the kinds of animals that these two boys encounter while at the circus. "What if" can focus on the kinds of acts at the circus as well as the kinds of people the boys might run into while at the circus. You can get as specific as you want or keep the story purposely vague.
Find a story idea that you've been struggling with and see if you can come up with a list of "what if" options for it.
Does your story take place at the beach? In the mountains? In the city? Shake things up a bit. Put your character in a compromising situation. Lead your imagination on a wild chase.
Give yourself the freedom to come up with as many ideas as possible for your story then choose the one that most speaks to you. It's not about which idea makes sense, only that you give birth to the idea and follow through with plot points. Be extravagant. Be outrageous. Be creative.