The admin bar is displayed by default. Each user can turn off the admin bar by unchecking the Tool Bar check box in the profile page, called User Info Update. The admin bar could be turned off globally, i.e. for everyone, using PIE Register, a very important plug-in. This can be selected in Settings –> Overrides section. Selecting Do not show admin bar will turn off admin bar for everyone except admins.  

On the right panel, there is a Site Admin selection. When the admin bar has been turned off, either globally by the admin, or by an individual user, clicking this will turn on admin bar.