I know that the Ctrl+Alt+F* shortcuts go to virtual terminals. This worked fine up through 11.10. But in 12.04, all I get is a blank screen when using a Ctrl+Alt+F* shortcut; I can still go back to my X session, though, by pressing Ctrl+Alt+F7. What is malfunctioning?
I meet the same problem after update from 10.04 to 12.04. It is the problem of your video card driver. In my situation, after the update, the driver has been disabled and replaced by sth different. Go to the 'system setting', find the 'additional drivers', check if the correct video card driver is activated.