The reason why this problem occurs because users are generally carried out under the window partition operations (many users will choose to install window and ubuntu dual system), such as disk partitioning and merging, etc., affect the original partition information.
Solution as follows
Step 1
First use the ls command shows the ubuntu partition installation information:
grub rescue>ls
Usually list all disk partition information, such as (hd0, gpt1) (hd0, gpt2) (hd0, gpt3) etc.
Step 2
Execute the following command:
grub rescue>set
Grub will list the current settings, such as prefix=(hd0,gpt1)/boot/grub,root=hd0,gpt1 or prefix=(hd0,gpt1)/grub,root=hd0,gpt1, precisely because these settings wrong , which leads to grub can not find the Ubuntu partition, followed by re-setting grub.
Step 3
If you know what ubuntu is installed on a partition, such as (hd0, gpt1), respectively, execute the following command
grub rescue> set root=hd0,gpt1 grub rescue> set prefix=(hd0,gpt1)/boot/grub (In some cases would be set prefix=(hd0,gpt1)/grub)
Step 4
Execute the following command:
insmod normal
If you still get error: unknown filesystem then back to step 3 grub settings, if there is a highlighted grub rescue>, represents the partition settings are correct, and then input normal. But here is not over, because if you reboot the system again, it will still encounter error: unknown filesystem this question, then you have to repeat the steps to see the appeal system selection interface, so there is a step needs to be done.
Step 5 Fix grub partition information
Choose ubuntu, start after entering the terminal, execute the following command
$ sudo update-grub $ sudo grub-install /dev/sda