You’ve got your protagonist planned out. You know your setting and the premise of your story. Perhaps you’ve written a good portion, with tension and a perfect adversary.
Then you bogged down, not knowing where to go next.
One way is to insert an old acquaintance from the character’s past.
This person needs to be a bit quirky or argumentative or a bully. She could have stolen the protagonist’s love interest or bested her in a competition.
Perhaps he’s the ex-boyfriend or husband, moving back home.
Write the story of what happens as soon as your character is aware of this person’s reappearance.
Use narrative and dialogue. Include emotional reactions: think sweaty palms, heart palpitations, trouble breathing.
Ramp up the tension. Perhaps there’s a nasty interaction, or your character overhears him saying nasty things about her.
Have fun with this one.