Dear Anya,
Hello! Welcome to the Microsoft Community. I'm glad to be able to assist you. If I misunderstand your question, please let me know promptly.
I understand that you seem to have lost the text and layout of your original Word file after using the online version of Word. I understand the trouble this problem has caused you, and I will try my best to help you solve this problem.
First of all, I want to confirm whether the "Word online" you mentioned refers to the web version of Word? How did you open your document in the web version of Word? I tried to open a custom document, and its format did not change. I used "File", "Open Project". And it will prompt you to upload this file to OneDrive and open it in a new tab. If you can't see it, you can check all your documents or OneDrive.
You mentioned that you created a new file with a book layout, but you can't use it normally. Can you tell me the specific situation? For example, you can't edit it. Did you encounter any error messages or prompts? It would be better if you can share a picture that can illustrate the problem.
I hope the above information is helpful. If you have any progress or questions, please let me know.
Best Wishes
Pyke.D | Microsoft Community Support Specialist