I recently updated a few machines to 11.10. On one of them, the promt to chose between gdm and lightdm. I tried selecting light dm, but the upgrade tool inevitably returned to the same prompt. I then chose gdm and the upgrade completed.

But now when I tried to boot, the computer presents no login screen. I'm thinking that it's because GDM was uninstalled in the upgrade process.

How can I install lightdm or gdm without logging in traditionally?

PS: apt-get does not seem to work in recovery mode.