修复 Ubuntu Grub

Open Terminal

Enter: sudo fdisk -l
Search ID=83,and remeber the sd[number],like sda3，If exist multiple 83, please verify which is / 。

switch root

1. Enter: mkdir /media/tmp
2. Mount / mount to the new directory: mount /dev/sda5 /media/tmp

   If the previous system /boot is mounted separately,you need to find /boot partition like sda6，then:
   mount /dev/sda6 media/tmp/boot

install grub

`grub-install --root-directory=/media/tmp /dev/sda`

If you see Installation finished，no error occured. successful.