Primary solution: Cinnamon 2.6 Update breaks menu and exit button.
To solve missing shutdown botons, use dconf-editor, under org.cinnamon.desktop.session set session-manager-uses-logind to true then logout and back in again and shutdown button should be working.
To resolve broken menu, simply right click on the menu, go to all the settings, choose and select theme. Menu repaired.
Alternatimamente and until this is resolved definitely, we can save the configuration menu applet to restore it if necessary.