When a section in Tangelo is in use it will be locked, no changes are possible for other users. This is visible in the document structure via a lock icon, see below. When the cursor/mouse is held over this lock, it becomes visible who has created the lock:
Usually the lock is removed automatically when leaving the section but sometimes (for instance due to a browser crash or using the back button instead of the exit button) the lock stays active. In those cases the account who locked the section or another user with document MANAGER privileges can resolve this issue. See picture below, use the option in the submenu (ref. 1) of the particular section to force unlock the section (ref. 2).