I suggest that you
Save (make a backup copy of) your personal files from the problematic persistent live system to another drive. You may need to boot from another drive for this purpose, and in that case find them in the partition for persistence in the directory tree /mountpoint/upper/ (and/or if you used mkusb-dus to create the persistent live system, in the NTFS partition labeled usbdata).
Create a fresh version of a persistent live system. This will overwrite the drive so you must save what you cannot afford to lose before doing it.
Restore your personal files to the persistent live system (typically into subdirectories of your home directory).
If necessary, install some program package, that you need.
Finally, if you used mkusb-dus to create the persistent live system, backup the content of the partition for persistence. Then you will be able to recover quickly from problems like this one.
Otherwise, use your own method to make a backup copy of the content in the partition for persistence (for example with tar or rsync).
And please remember to backup also the content of the NTFS partition labeled usbdata (if there is such a partition).
Edit: Here is a correct version of grub.cfg for a persistent live drive of Lubuntu 18.04.5 LTS made by mkusb-dus from lubuntu-18.04.5-desktop-amd64.iso. It should be copied to the third partition of the persistent live drive mounted at 'mountpoint': /mountpoint/boot/grub/grub.cfg
set timeout=10
set default=0
if loadfont /boot/grub/font.pf2 ; then
# set gfxmode=auto
insmod efi_gop
insmod efi_uga
insmod gfxterm
# terminal_output gfxterm
fi
set menu_color_normal=white/black
set menu_color_highlight=black/light-gray
if loadfont /boot/grub/font.pf2 ; then
# set gfxmode=auto
set menu_color_normal=light-gray/black
set menu_color_highlight=black/light-gray
set color_normal=light-gray/black
set gfxmode=800x600
insmod efi_gop
insmod efi_uga
insmod gfxterm
insmod ieee1275_fb
insmod vbe
insmod vga
insmod video_bochs
insmod video_cirrus
terminal_output gfxterm
insmod png
background_image /mkusb-grub-bg_800x600.png
fi
menuentry "Run Lubuntu - persistent live" {
search --set=root --fs-uuid 2020-08-06-23-08-05-00
set gfxpayload=keep
linux ($root)/casper/vmlinuz file=/cdrom/preseed/lubuntu.seed boot=casper quiet splash fsck.mode=skip persistent ---
initrd ($root)/casper/initrd
}
menuentry "Run Lubuntu - persistent live to RAM" {
search --set=root --fs-uuid 2020-08-06-23-08-05-00
set gfxpayload=keep
linux ($root)/casper/vmlinuz file=/cdrom/preseed/lubuntu.seed boot=casper quiet splash fsck.mode=skip toram persistent ---
initrd ($root)/casper/initrd
}
menuentry "Run Lubuntu - live-only to RAM" {
search --set=root --fs-uuid 2020-08-06-23-08-05-00
set gfxpayload=keep
linux ($root)/casper/vmlinuz file=/cdrom/preseed/lubuntu.seed boot=casper quiet splash fsck.mode=skip toram nopersistent ---
initrd ($root)/casper/initrd
}
menuentry "Try Lubuntu without installing" {
search --set=root --fs-uuid 2020-08-06-23-08-05-00
set gfxpayload=keep
linux ($root)/casper/vmlinuz file=/cdrom/preseed/lubuntu.seed boot=casper quiet splash ---
initrd ($root)/casper/initrd
}
menuentry "Install Lubuntu" {
search --set=root --fs-uuid 2020-08-06-23-08-05-00
set gfxpayload=keep
linux ($root)/casper/vmlinuz file=/cdrom/preseed/lubuntu.seed boot=casper only-ubiquity quiet splash ---
initrd ($root)/casper/initrd
}
menuentry "OEM install (for manufacturers)" {
search --set=root --fs-uuid 2020-08-06-23-08-05-00
set gfxpayload=keep
linux ($root)/casper/vmlinuz file=/cdrom/preseed/lubuntu.seed boot=casper only-ubiquity quiet splash oem-config/enable=true ---
initrd ($root)/casper/initrd
}
menuentry "Check disc for defects" {
search --set=root --fs-uuid 2020-08-06-23-08-05-00
set gfxpayload=keep
linux ($root)/casper/vmlinuz boot=casper integrity-check quiet splash ---
initrd ($root)/casper/initrd
}
If there are other damages in the persistent live system, or if you created it by some other method (not by mkusb-dus), it will not help to use this grub.cfg file, and I suggest again that you create a fresh version of a persistent live system.
Edit 2: Try first without editing.
If there will be no graphics, you may have problems with the driver for the graphics chip/card. In that case try first to add nomodeset near the end of the line starting with 'linux' (in each menuentry in the file grub.cfg),
linux ...... nomodeset ---
Next you could try to replace nomodeset with your string like so:
linux ...... amdgpu.ppfeaturemask=0xffffffff ---
This may or may not work, because it may need some driver that is not available in a live or persistent live system.
The dots, ......, represent some text content, for example the first instance of line should be
linux ($root)/casper/vmlinuz file=/cdrom/preseed/lubuntu.seed boot=casper quiet splash fsck.mode=skip persistent nomodeset ---
and/or
linux ($root)/casper/vmlinuz file=/cdrom/preseed/lubuntu.seed boot=casper quiet splash fsck.mode=skip persistent amdgpu.ppfeaturemask=0xffffffff ---
