You've got this great idea to start a story off. It may be the location, the first few lines of dialogue between friends, friction between two antagonists, or a host of other beginnings, then - you just run out of steam. Who hasn't been there? Here's an idea or two of how to get past that first stumbling block and progress your story's plot.
First, ask yourself what it was about those first few words that got you excited? Was it the characters themselves, was it the dialogue between them, or maybe the sparks between them. Try to nail down in your own mind just what it was because this can be the key to getting on with the plot.
Let's examine these two examples.
Dialogue/friendship
A couple of characters have presented themselves to you and you've got some dialogue down. You're delighted with the way they interact and you feel a glow of satisfaction at the scene so far. But it isn't going anywhere. You are reluctant to tamper with their relationship but the storyline has hit the buffers. In that situation lies your answer.
At this point you have to bring in something which interrupts the status quo. It may be some innocuous bit of friction between them that grows and threatens to tear them apart, or it may be something from outside their friendship that either pulls them apart or brings them closer because of a shared threat. Don't worry if it's the former - they can come together again in the finale.
Location
You've got this amazing location, real or imaginary, that really moves you. It may be exotic, idyllic, wild, peaceful. Whatever it is, you feel an emotional thrill and think, "I could write a story about this". You get the first two paragraphs or so down on paper and the muse takes a holiday. What to do?
Similarly, as with the friendship scenario, introduce something that radically alters that first emotive impression. In a romantic setting it could be an argument that ends in tradgedy, in a mountain landscape it could be two rock climbers lost and frightened - if only temporary - in such an inhospitable spot.
In both cases, the secret is to move away from the first impression you present to the reader. This can be done in a multitude of various ways. Just use your imagination and ask yourself 'what if...' - it will soon fall into place.
Think of the classic senario: boy meets girl, boy looses girl, boy gets girl back again. It's the same thing. Break up the first emotive scene, take your characters through hell and high water, but then bring them back to the first situation. And there you have your plot!