What does Judy mean when she tells Jess and Leslie that she is stuck?

Judy means that she has currently run out of ideas on how to proceed in one of her novels.
In the story, Judy is Leslie's mother; she is a novelist, writing under the pseudonym of "Judith Hancock." When Judy is writing, she often seems distracted. In chapter 9, the rain threatens to spoil Leslie and Jess's week-long vacation. The two children spend a short time moping on the porch of Leslie's home. Eventually, Leslie admits that she wants to visit Terabithia, even though it is raining.
Jess concurs, and Leslie begins to collect her boots and raincoat. She suggests that they go to Jess's house to collect his as well. However, Jess says that he does not have boots or a raincoat that fit him at home. He insists that he will go to Terabithia just as he is. Leslie then suggests that she will fetch one of her father's old raincoats for Jess.
Just as Leslie is making her way upstairs, Judy appears in the hallway. She seems preoccupied, and Leslie immediately apologizes for disturbing her mother. Judy replies that she is "stuck" anyway and may as well stop for a break. This means that Judy is out of ideas on how to proceed on one of her stories. Some people will say that she is suffering from a mental block.
After a brief, distracted conversation with the children, Judy eventually returns to typing furiously on her typewriter. This prompts Leslie to comment that her mother has come "unstuck."