I am not a writer myself, so take this with a grain of salt, but it might be better to just start writing things down in a short framework, even if elements of the story are missing or incomplete. Like you may think about a specific scene/situation so you can jot that down, then you might write something like “there would be a problem that caused X character to be here” just to have a placeholder so later you can come back and replace it with something.

Also consider that studies have shown physical activity can help stimulate your mind. So maybe go for a walk to think about your story or what you want from it