How can I repair a Ubuntu installation that is missing numerous packages?

Question

For an unknown reason, my Ubuntu workstation decided to do a partial upgrade to Oneiric (I had originally ignored the upgrade). After several reboots and running

apt-get update
apt-get upgrade
apt-get dist-upgrade

each time, I was finally able to boot my system and have X start GDM as normal. However, I noticed that a lot of packages were missing, including the default theme, firefox, and who knows what else. I've already lost my whole afternoon, so how can I get my computer back to where it was? Why in the world did this happen?

Answer 1

I suggest to install (or remove and install, if already there) the package ubuntu-desktop.

Generally this package take with it almost everything needed to have a standard Ubuntu installation.

Answer 2

I had a similar problem, but I installed Gnome 3 knowing that there was a high possibility that it will mess up the login. Insert the Live CD/USB for your current version, and boot from it. Click "Install Ubuntu". There should be four options there. Click the second one (I think it says "Upgrade Ubuntu xx.xx to Ubuntu xx.xx"). It will keep all your personal files, but will reinstall all the default applications.