Note that if the chapter is locked by you, you can unlock it yourself without being an administrator. More information about how to do so can be found here.
Select the project, click on the "Chapter" menu and select "View/edit chapter":
You will see a list of all of the chapters in the project.
Chapters that are locked are indicated by a red padlock icon. To unlock the chapter click on the padlock.
If a user is editing this chapter they will no longer be able to save their work. You should only unlock a chapter if you have been specifically asked to do so and have the consent of the user who has the document checked out.
After clicking on the padlock you will be asked to confirm by clicking on the "Unlock" button.
The chapter is now unlocked and is available for users to access.