I don't know how it happened, but when I restarted my laptop I got an error saying:
Xsession: unable to launch "gnome-session --session=ubuntu" X session --- "gnome-session --session=ubuntu" not found; falling back to default session
I then found this post:
I went to terminal more with ctrl+alt+f1 and reinstall ubuntu-gnome-desktop. It complained that it could not download ubuntu-release-manager update-manager, and update-notifier, but wasn't going to install them. When I tried
sudo apt-get install update-notifier it told me to download update-notifier-common; when I tried doing that is told me I was missing gcr, debconf, default-dbus-session-dbus, dbus-session-dbus, and more. But, when I tried installing these it said they were already up to date.
Then my laptop ran out of power. When I turned it back on I realized it could not connect to the internet.
I just got my laptop two months ago, every thing I care about is backed up on github/google, I just really need to be able to use if for school. Its on ubuntu 16.04 lts.