I am using Ubuntu 12.03 and the system recently installed some updates. Then I logged back in, the top and bottom panels (System, Time, etc) were missing from gnome. I thought it might be an issue with the graphics drivers so I updated my nvidia drivers to no effect using the following commands:
sudo apt-add-repository ppa:ubuntu-x-swat/x-updates sudo apt-get update sudo apt-get install nvidia-current
When I take a screenshot, the panels appear in a duplicated screen to the right that I cannot see on my screen. Ctrl+Alt+Left/Right do not change the workspace.