Sometimes (rarely) it happens that Slingshot launcher is neither accessible by shortkey or by clicking Applications on panel.
How to force it to restart without logging off?
The parts of the desktop like Gala (the window manager), Wingpanel, Plank (the dock), and Slingshot (the apps menu) are automatically restarted by a process called Cerbere.
You'll only need to end the process for Slingshot and Cerbere will automatically restart it. You can do this in Terminal (shortcut: ⌘ + T) with:
The process that Cerbere keeps running in background is
slingshot-launcher --silent so I guess that command might do the trick.