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?