Ubuntu 12.04 and initially choosing normal Ubuntu at login gave me
Ubuntu 3D. However, for a few days now, selecting either
Ubuntu 2D gives me
~$ echo $DESKTOP_SESSION ubuntu-2d
Your graphic card is not supporting the 3d mode.
I may be a trouble with your graphic drivers. It happens often after a kernel update. (even more if it's an ati or nvidia graphic card)
You can confirm this with this command : glxinfo | grep rendering (install glxinfo if you don't have it)
You may need to reinstall your graphic driver if it's nvidia or ati.
Leave me a comment if you need more explanations.
If you have a laptop with hybrid graphics it may also arise the issue you experience. Then you might want to look at the Bumblebee project which enables your hybrid graphics to work correctly.