It's convenient to boot your system with .iso when your Linux can't boot up normally.

First, download a .iso and put in a partition. For example, Deepin 15 one

$ wget http://cdimage.linuxdeepin.com/releases/15.1.1/deepin-15.1.1-amd64.iso -O /deepin-15.1.1-amd64.iso

Second, edit /etc/grub.d/40_custom, add a menu entry to it

menuentry "Deepin 15.1" {
        set isofile="/deepin-15.1.1-amd64.iso"
        loopback loop (hd1,gpt2)$isofile
        linux (loop)/live/vmlinuz.efi boot=live iso-scan/filename=$isofile noprompt noeject
        initrd (loop)/live/initrd.lz
}

If the .iso is an Ubuntu one, replace live with casper.

Further reading