gnome - Unity's gone! How do I get it back?

Earlier today my Unity desktop disappeared: I got a black screen with white text, but it disappeared too quickly for me to read. When the desktop reappeared, it was the Ubuntu Classic desktop. I used $ unity --reset but that did not do anything. I tried rebooting so I could choose unity from the list when I logged in, but although there was Ubuntu choice, there was no unity listed (but classic was listed). I was able to install Unity 2D and am using that, but really want to get 3D back.

I had been using Ubuntu without problems for several weeks when this happened. My graphics card is onboard a Dell Latitude desktop - a couple years old - and is reported as an Intel G33/G31.

I've looked at other requests for help here, and tried suggestions when they seemed to relate to similar problems, but nothing seems to work so far.

Any ideas? Thanks!

This is part of the output of my latest attempt to run unity --reset

Window manager warning: 0x3e01c35 () appears to be one of the offending windows with a timestamp of 1309472834.  Working around...
Window manager warning: last_user_time (1309473695) is greater than comparison timestamp (1126160).  This most likely represents a buggy client sending inaccurate timestamps in messages such as _NET_ACTIVE_WINDOW.  Trying to work around...
Window manager warning: 0x4c0046c (mdk@Habane) appears to be one of the offending windows with a timestamp of 1309473695.  Working around...
Window manager warning: Received a NET_CURRENT_DESKTOP message from a broken (outdated) client who sent a 0 timestamp
Window manager warning: Buggy client sent a _NET_ACTIVE_WINDOW message with a timestamp of 0 for 0x4c0046c (mdk@Habane)
Window manager warning: meta_window_activate called by a pager with a 0 timestamp; the pager needs to be fixed.

I think you can bring up a terminal w/ CTRL+ALT+'T', then use apt-get to install unity,

"sudo apt-get install unity"

if you haven't tried that already...