iMac (21.5-inch and 27-inch, Late 2012), MacBook Pro with Retina display: Menu bar may not appear on first login
You may notice that the menu bar does not appear the very first time you log in to a user account on your computer (or after reinstalling Mac OS X). This may occur on iMac (21.5-inch and 27-inch, Late 2012), MacBook Pro (Retina, 13-inch, Late 2012 and later), and MacBook Pro (Retina, 15-inch, Early 2013) computers.
Simply restarting the computer will resolve this issue. You can do this by pressing Control-Eject to restart, or by pressing Control-Option-Command-Power to shutdown your Mac.
You can learn more about using keyboard shortcuts in this article.
You may notice that using trackpad gestures to switch to a different desktop will sometimes cause the menu bar to appear if this issue occurs.