I am running Natty Alpha 3. I used Update Manager to install something (not sure what it was), then I restarted, but instead of listing all the kernels, it gave me a command line. I don't know what happened and how to use it. Help!
May I suggest you to recover your grub installation with a live cd?
I could recover grub many times by following these instructions at: https://wiki.ubuntu.com/Grub2#Recover
"METHOD 3 - CHROOT
This method of installation uses the chroot command to gain access to the broken system's files. Once the chroot command is issued, the LiveCD treats the broken system's / as its own. Commands run in a chroot environment will affect the broken systems filesystems and not those of the LiveCD.
1) Boot to the LiveCD Desktop (Ubuntu 9.10 or later). Please note that the Live CD must be the same as the system you are fixing - either 32-bit or 64-bit (if not then the chroot will fail).
2) Open a terminal - Applications, Accessories, Terminal. 3) Determine your normal system partition - (the switch is a lowercase "L")
sudo fdisk -l
If you aren't sure, run
df -ThLook for the correct disk size and ext3 or ext4 format.
4) Mount your normal system partition:
Substitute the correct partition: sda1, sdb5, etc.
sudo mount /dev/sdXX /mnt# Example: sudo mount /dev/sda1 /mnt
5) Only if you have a separate boot partition: sdYY is the /boot partition designation (for example sdb3)
sudo mount /dev/sdYY /mnt/boot6) Mount the critical virtual filesystems:
sudo mount --bind /dev /mnt/dev sudo mount --bind /dev/pts /mnt/dev/pts sudo mount --bind /proc /mnt/proc sudo mount --bind /sys /mnt/sys7) Chroot into your normal system device:
sudo chroot /mnt
8) If there is no /boot/grub/grub.cfg or it's not correct, create one using
9) Reinstall GRUB 2:
Substitute the correct device - sda, sdb, etc.
Do not specify a partition number (e.g. do not specify sda5, instead it could be sda)
10) Verify the install (use the correct device, for example sda. Do not specify a partition):
sudo grub-install --recheck /dev/sdX11) Exit chroot: CTRL-D on keyboard 12) Unmount virtual filesystems:
sudo umount /mnt/dev/pts sudo umount /mnt/dev sudo umount /mnt/proc sudo umount /mnt/sys
13) If you mounted a separate /boot partition:
sudo umount /mnt/boot
14) Unmount the LiveCD's /usr directory:
sudo umount /mnt/usr
15) Unmount last device:
sudo umount /mnt
It happened to me too, on a regular upgrade. Thankfully I was working on the dev system, determining the stability of the upgrade before pushing it to production.
The answer from "steabert" is not helpful at all. That's the answer for people who run Windows. Early releases are for finding bugs, and Cg2916 found one.
I recovered by using the grub console to continue booting, then reinstalled grub after I was able to log in.
set root (hd0,1) linux /vmlinuz root=/dev/sda initrd /initrd.img boot
Lots of fun boot stuff happens
sudo grub-install /dev/sda
Of course, you'll need to adjust for correct partition numbers and device names, but it's fairly painless after you get over the initial pain.
Now, how do I do this on a virtual machine in a remote location where I only have SSH access...