Now, when you say middle, do you mean middle as in the chunk of the story in between the beginning or the end or middle as in the part of the story directly after whatever you already have?
If after what you already have, just keep in mind recent events and characters and think how they would continue to act as said events progress and write that.
If you're talking the whole middle of the story then... Yeah, that's difficult. You'll want to come up with some characters, places and events that you can look forward to writing and intermesh with your idea of the storyline. You'd be surprised at how much coming up with random characters can advance the plot. Now, i'm not saying you half-arse things, but tbh if you don't know what to do next just have a new character come in or chuck some weird enemy at them.
I made an old dude selling fish pop up and he turned into a badass retired navy captain or something who became an avenue to introduce the next big bad guy in the story.