well sins i had this problem and i fixed it i think i can help you. I think its a problem that happens sometime during the installation i don't quite know what causes this but i re-installed GNOME and my meunu started working again. I would suggest doing this. you can do this via click on terminal and typing
sudo apt-get install gnome
it should work, did for me
hope it works let me know what happens and if the problem still remains.