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 look at a couple of 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 don't want to break up their relationship but no storyline presents itself. 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 initial description down and it all comes to a stop. What to do?
Again, as with the friendship above, bring into the scene something that changes that first emotive impression. In an idyllic setting it could be a clifftop struggle that ends in tradgedy, in a wild landscape it could be a couple of runaways finding shelter in a derelict cottage and making a home - 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!