If you are unable to access Unity 3D (system loading in to Unity 2D as by default) it may be worth doing the following which may sort the issue.
Open up System Settings by clicking on the power cog at the top-right of your screen, then select 'System Setting...' from the drop-down menu':
A window will open up where you will need to select the 'Details' icon:
The next window will display some basic info about your system; Memory, Processor, Graphics, OS type and Disk:
If your system Graphics has a value of unknown Like in the image of my system above the following steps may be able to fix your issue:
Install a package called mesa-utils to add OpenGL support to your system by running this command in the terminal(Ctrl+Alt+t):
sudo apt-get install mesa-utils
At this point I also needed to remove the nvidia-current package using this command:
sudo apt-get remove nvidia-current
Restart your system, when you log back in you should hopefully be in Unity 3D.
Now when you when you look in details in the system settings you should see something like 'Graphics Intel® Sandybridge Mobile'.