Rescue System

If your system can't boot up, you can use the "Enable/disable rescue mode" feature to access the hard disk. 

The configured rescue system runs Rescatux. Once enabled, boot into it and locate the original hard drive by typing:

lsblk

You'll then see the hard disk devices, such as /dev/sda1, which should match your disk's capacity (for example, 50 GB).

Next, simply mount the device to a folder like /mnt using:

sudo mount /dev/sda1 /mnt

Make any changes you need.

When you're done, disable rescue mode.