A family of Microsoft word processing software products for creating web, email, and print documents.
Is it the button or the keyboard shortcut that isn't working? If the latter, have you installed any new software recently? It is not at all uncommon for third-party software to hijack Word's keyboard shortcuts. If you're lucky, there will be a Settings or Options or Preferences setting in the third-party software that will allow you to reassign the shortcut (or at least disable it). If not, at least you can assign a new shortcut (check first to see whether Ctrl+Shift+M is still assigned to it).