I read the comments and suggestions just now and they're good. Yep. Good advice. So why do I imagine you sitting there, staring at your screen, thinking, I know this shit. I can't do it.
I wrote a novella: 17,000 words and reality hit me like a damn sledgehammer: I'm going to play hell selling this thing as is. I need to make it longer. I need to write at least 60,000 words. So I kicked it around for a while. I realized that I was kind of paralyzed by the idea of fleshing out this--whatever it was.
My problem was, I wrote the thing and I thought it was done. I resisted considering that it wasn't. Then I thought, what if. . .
That's the key, I think. What if. Look at what you wrote, and think to yourself, what if A happened? What if B did this? What if X, Y, or Z happened before. It doesn't hurt to kick around some what ifs, see if anything clicks. Another idea: Think of the story you'd like to read, the characters, plot, give yourself permission to consider alternate plot lines. Maybe another character thrown into the mix. Alternative endings, alternative beginnings.
Don't sell yourself short. You have a great imagination, right? Give yourself permission to consider the possibilities.