I had pretty much the same problem under 12.04. Unity was slow with my aged GPU, so I was using Unity 2d as my desktop for months. For no apparent reason, it suddenly refused to display launchpad and title bar (for all users). Reenabling Unity in CCSM as sugggested in the thread referred to by OP was part of the solution, but none of the other measures suggested here, there or in several other related threads (including, but not limited to:
Reinstalling ubuntu desktop
Removing all relevant (e.g. compiz, gconf, etc.) configuration information
helped me to completely resolve these symptoms under 12.04 (64 bit).
would not work for me. After delivering a variety of error and information messages, it would always hang at:
Setting Update "run_key"
as shown in the log posted in the thread titled "unity has vanished" for unity run with no arguments.
I noticed that the error message delivered by:
was similar to some of the errors reported by unity --reset:
error while loading shared libraries: libGL.so.1: cannot open shared object file: No such file or directory.
After extensive searching, I found the following solution:
Get the path of libGL.so.1 by using the command locate libGL.so.1.
Add a link to the library in /usr/lib/ as shown in the following example:
sudo ln -s /usr/lib/i386/mesa/libGL.so.1 /usr/lib
(courtesy of J.D. Bartlett)
Restart the computer.
This not only allowed both unity_support_test-p and unity --reset to run, it also allowed Unity 2d to start. Like the author of the above-mentioned post, I have no idea what caused my problems. I am also still not sure whether the link is a complete solution, or whether I should reinstall the graphics libraries completely, as described by Cynical here.