I just updated one of my VMs from Ubuntu 10.10 to 11.04 Natty Narwhal using the Update Manager. All seemed to go well during the upgrade process. When it rebooted for the first time however, I was left with a grub prompt rather than a booting system. Grrrrrr.
NOTE: The following assumes the default disk layout. If you installed to a different disk or partition, you’ll have to adjust the steps below accordingly.
The fix is to manually boot the system at the grub prompt by typing
linux /boot/vmlinux-2.6.38-8-generic root=/dev/sda1 ro
Then once you are successfully booted, re-install grub like this:
sudo grub-install /dev/sda
Thanks to Rob Convery for the tip!