After upgrading to 13.04 Unity interface is not showing

This worked for me:

1. Press Ctrl+Alt+T to open a terminal window.
2. At the terminal prompt, type ccsm. If the CompizConfig Settings Manager window appears, skip to step 5. If not, continue with step 3.
3. Type sudo apt-get install compizconfig-settings-manager. Type your password when prompted.
4. At the next terminal prompt, type ccsm.
5. In CompizConfig Settings Manager, select the check box for the OpenGL plugin if it isn't already selected.
6. Click the Ubuntu Unity Plugin item.
7. Select the Enable Ubuntu Unity Plugin check box, if it isn't already selected. If it is already selected then uncheck this and again select it.
8. Exit from the CompizConfig Settings Manager.
9. Press Ctrl+Alt+Delete. When asked, confirm that you want to log out.
10. I did a cold reboot at this point. I'm not sure it's necessary, but I'm a bit paranoid.

After this, Unity came up as it's supposed to for me.

I have a 64-bit AMD Phenom CPU and an nVidia GPU; I'm running 64-bit Ubuntu with the nvidia-current driver set, and I've just upgraded to 13.04 (at which point Unity disappeared).

I fixed this issue by opening a terminal with Ctrl + Alt + T, resetting compiz with dconf reset -f /org/compiz/ and logging out with gnome-session-quit.

When I logged back in the interface was there again.