Word is a customizable program that allows you to display different tools for your project. The program has a number of tools that you can use; however, it also gives you the option of choosing a select few of them to create a customizable toolbar. Sometimes this customizable toolbar can vanish causing much frustration.
The common reason behind it disappearing is that the menus have deleted your toolbar. You probably need to make another one in order to have it on the display like earlier. Often it is much better to create multiple versions of the same toolbar, which will act as back up, in case one is deleted.