If you're creating your book with collaborators and a page has a small padlock icon in the spread browser, it means someone else is working on that page. Once they're finished, it will unlock. If you're the author, you'll then get to approve a collaborator's additions.
If you're not working with collaborators, but the word "locked" is appearing on your page, it may just take a few seconds to complete a process before you can continue editing. Usually, turning a page and/or saving your book will unlock it. If you are still unable to edit a page, save and then exit your book, then reload the project from My Books.
In the unlikely event you're still experiencing difficulty, drop us a line and our technical team will look into things for you.