Okay, so, if all you have for an idea is a situation or scene, how do you get to a real story idea, one in which something happens to somebody and so what? Well, remember the elements of fiction, the basic parts of a story? I mean the most basic parts that you may have studied in middle or high school, not more involved, elaborate ones from college or graduate school or...you know...somebody's blog? I'm talking character, setting, point of view, theme, and plot. Every story has these elements/parts, and sometimes if you can work on developing those, you can use the material you generate to get to what happens to someone and why anyone would care.
Character--Try working on some characters, particularly the one who might end up being your protagonist. You can end up changing your mind about this. You can find various charts to help you do this. Some people suggest that these charts go over the top. How much does anyone need to know about another human being, especially one who only exists in your mind and who you might end up deep-sixing before you start the writing? There's also the possibility of overwhelming yourself with detail. I used character charts for a couple of books, but quite honestly, I haven't sold them. But beyond that, yes, working on characters can help you determine what these people have to do with your original situation or scene.
Some things to consider: How this character may change because of the scene, it's impact on him or her. Also, you can't go wrong dwelling on the classic journalistic questions of Who, What, When, Where, Why in relation to this character, not the story as a whole. Who is this person, of course, but additionally what is she doing in the scene/situation, when is this scene/situation going down, where, and, most importantly in my humble opinion, why.
This should give you some material that will intersect with the other elements when you start working with them. More on that next weekend.