My menu bar disappeared

Has it disappeared before, unless running some App in Full Screen mode it should always be there.

Apple Menu Bar Icons Gone...

Safe Boot from the HD, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions.

PS. Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive

You've probably got a couple of corrupt preference files.

Move these files to the Desktop for now...

/Users/<yourname>/Library/Preferences/com.apple.systempreferences.plist

/Users/<yourname>/Library/Preferences>com.apple.systemuiserver.plist

/Users/<yourname>/Library/Preferences/ByHost>com.apple.systemuiserver.xxx.plist

where 'xxx' is a 12 digit (hexadecimal) numeric string.

Reboot.

This will reset your Menu Bar to the default, you'll need to go through System Preferences to reset the ones you need. (If they are already checked, uncheck them first and then recheck them).

You unintentionally changed your current app to Full Screen Mode. The Menu Bar will appear if you move your mouse pointer to the top of the screen. You can exit the Full Screen Mode by hitting the Escape key.