Here's a Premise/Story in Need of an Ending:
We're in a diner. Our clock narrator--yes, an inanimate object is telling the story--tells us life is very routine and boring in the diner. But today there's a couple worth looking at. He's been crying. She's determined to break up with him. The man says something very rude. She slaps him. Some of the wait staff and the patrons pretend not to notice. Finally, an older woman gets up and approaches the couple's table and she brings the story to its end.
So....what happens?
When you finish your story you want your reader to think, Wow, but of course. In other words try to come up with an ending that feels unexpected but somehow inevitable.
Here are a few concepts for ending a story:
Circle back to the beginning
Some artful verbal repetition.
Reconsider an object
Look to the future
What is in store for your characters? What problems do they still face?
Have you already written your ending?
There's no prize for going on too long.
Here are some pitfalls to avoid:
Tricking your reader (for example, it was all just a dream, he's really crazy, etc...)
Deus ex Machina (Comets kill everyone, a random serial killer does in your protagonist, etc..)
Wrapping things up too neatly
Moral lessons
Solving everyone's problems
Not knowing your story well enough to craft a satisfying ending