Today my PC stopped logging into the Unity environment, falling back to Unity 2D. All the nice special effects have gone too.

Typing unity into a terminal produces the following output:
Checking if settings need to be migrated
Checking if internal files need to be migrated
Backend : gconf
Integration : true
Profile : unity
Adding plugins
Skipping upgrade com.canonical.unity.unity.01.upgrade
Skipping upgrade com.canonical.unity.unity.02.upgrade
Initializing core options...done
Initializing bailer options...done
Initializing detection options...done
Initializing composite options...done
Compiz (opengl) - Fatal: Root visual is not a GL visual
Compiz (bailer) - Info: Ensuring a shell for your session
laurie@laurie-desktop:~$ Window manager warning: Log level 16: Theme directory emblems/16 of theme FS_icons has no size field
How to I go about diagnosing and resolving this issue?