Many of you left that post asking, "But what if I don't know how I want the story to end?" That is an extremely valid question, and is one that all of us as authors have asked ourselves. Most of the time, we've asked "What happens next?!" in a moment of frustration.
Take a breath. You're not alone, and you'll get there.
In those moments, my advice to you is to go back in to your story and work on editing. Not only is it work that needs to be done anyway, but it will also force you to spend more time with your characters. Listen to their conversations, watch how they respond to things, and make sure all of those details are how you want them.
What will happen is that you will get to know them better.
Think of your characters as friends that stay with you everywhere you go. They can be kind of like roommates that you can't shake. As you spend more time with them, you begin to be able to anticipate how they would react to different situations.
Have you ever heard someone say, "Don't do that, they'll freak out!" when hearing that something is about said or done to someone else? Then you know what I mean. We come to understand how our friends will react in certain situations, and your characters are no different.
As you get into your character's heads a little more, go back to where you left off and ask yourself, "What would they do here?" Maybe you have had an idea of where you'd like the story to go, but aren't sure how to get there. In that instance, ask yourself, "What would make them do/say that?" Again, imagine that they are someone you actually know in real life. We do this with our friends and family all of the time. Picture what they would do in that instance, and then run with it.
You can always edit things later. Never forget that.
If you're still stuck, go back again. And again. Even if it is nothing more than allowing your mind to wander with the characters. Don't be afraid to imagine them in situations that have nothing to do with your story.
What would happen if the two main characters went grocery shopping together? What would they talk about?
If two characters despise each other, imagine them in a cab together stuck in traffic. What would happen?
Even if none of those moments make it into the story, what they do or say in those situations may come up in other places as your are writing. When they are left in a frustrating situation, what did they say while stuck in traffic? Is that reaction appropriate in the story? If not, what do you need to change?
Think about it and write it down. The story will come together.