[Ubuntu] invalid environment block

When starting the ubuntu with GRUB, that display some error below:

error: invalid environment block
Failed to boot default entries.
Press any key to continue...

Here is the way to fix the bug.

1. Make sure that you have a live cd in your hand.

2. Boot the computer, under the fixed model, insert the code below:

cd /boot/grub
rm grubenv
grub-editenv grubenv create
grub-editenv grubenv set default=0

3. Now you can check whether the option is ok.

grub-editenv grubenv list

That will display:

default=0

Since then, we fix the bug! Reboot the computer, and...

Have fun!