I have received this question a few times and decided to share it.
People using Microsoft Office 2007 have reported that all their buttons have disappeared. This is easy to fix but annoying that you have to do it in the first place. Essentially the person has double clicked on the tabs like home row and others. I do this when it the program is slow or I am not paying attention. All the buttons like bold and font size collapse leaving just the tabs. To bring them back it is just a matter of double clicking on any of the tabs again. I understand why this is here but it just gets in the way. So when you are staring at Microsoft Word wondering what happened give this a try.