I must have click something by mistake. Now when I enter info in a single cell in one page, it ends up in the same cell in all of the other pages. How can I undo this please? Thanks very much.
This should happen only if multiple sheets are selected. By default, every Calc file has three sheets. You can switch between them using the tabs at the bottom, left of the scroll bar. If more than one sheet is selected, all further changes are applied to all selected sheets. To end the "select-all" phase and select a single sheet, it should be sufficient to click on its tab (without holding SHIFT of CTRL keys).