Sound familiar? Believe me, it happens to all of us. But here's a few strategies that have worked for me to "unstick" a story:
1. Add a character. While you don't want to populate a short story with to many characters, sometimes a story suffers with too few. Your protagonist might need a love interest, a nemesis, or even a side-kick. If you decide to go this way, make sure it's a well-rounded character. A cardboard character could just make the situation worse.
2. Figure out why you wrote what you wrote. Did something in what you've written already travel from your subconscious writer brain right to the keyboard without passing through your critical editor brain? Good! It is probably some of the best material that you have on the page. Now you just need to think about why it ended up there. If it takes your story in a new direction--go with it.
3. Set it aside. Let your subconscious writer brain work on the story overnight, or even for a few days. However, don't set it aside for more than a week or so, or your subconscious writer brain will give up and start working on something else.
4. Submit what you have so far to your critique group. Don't feel like everything you submit needs to be finished. Sometimes, just hearing others talk about your story will give you an idea. (Don't have a crit group? Get one--either on-line or in person. They are critical).
5. Look at POV. This can be tough, but ultimately rewarding. Try switching from first person to third person limited. Or vice versa. Alternatively, try telling the story from the POV of a different character in your story.
6. Bang it out. If all else fails, decide you are going to finish the story even if you think it sucks. Make sure you get to the end, even if you are not happy with it. You might surprise yourself when you go back and look at it. Or you may find yourself at a point where 1 through 5 might work for you. Remember that your critical internal editor is your enemy early in the creative process. Don't let him or her stop you!