The workaround is to simply restart your computer, says the Mac maker
Apple is confirming a bug on Macs shipped Late 2012 which involves the OS X menu bar not displaying upon boot. The Cupertino giant has a simple resolution for the problem, though a patch will still be necessary in the future.On MacBook Pro (Retina, 13-inch, Late 2012), iMac (21.5-inch, Late 2012), and iMac (27-inch, Late 2012) models, the bug in question may cause the menu bar to remain hidden on first login.
According to KB article TS4440, “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).”
Apple says this may occur on the newest iMac models (21.5-inch and 27-inch, Late 2012) as well as the 13-inch MacBook Pro with Retina display, which also shipped late this year.
The “resolution” is to reboot, says the company - “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,” reads Apple’s documentation.
The Mac maker also directs customers to a separate knowledge base article (HT2448) where they can learn more about using keyboard shortcuts.
Under “additional information” Apple further notes that customers may notice that “using trackpad gestures to switch to a different desktop will sometimes cause the menu bar to appear if this issue occurs.”
This isn’t necessarily a solution to the problem, but Apple believes it can help.
The Cupertino giant fails to mention which versions of OS X exhibit the bug, but it is reasonable to assume that Mountain Lion is the most affected version, since this is the Mac OS that comes pre-installed on these Macs.