Please help. I uninstall compiz and it won't work please help :(
As mentioned in the comment above, this is standard gnome-shell behavior. If you need access to the menu you can type alt space and the menu should pop up. If you just want to close the application you click on it's name in the panel at the top and hit quit. If you just want it in a window again, you can click and hold in the menu bar and "pull" it out of maximize. You can change this behavior as well by editing the style sheets for the theme you are using. If you are using the default theme you can change this behavior by
and changing the value of has_title to true on this line.