After solving this problem I managed to get myself into truble again out of nothing by trying to change the display of the dual boot option page in Boot Customizer.

I have changed the background, the fonts size (I have increased them) and font style (I have chosen UnDotum). But Boot Customizer gave me an error (I mean a message that the application was closed unexpectedly or smth). I have restarted BootCustomizer and the settings were there. When I rebooted, instead of the normal boot options list, just the background image that I had selected and nothing else.

I used Boot Repair to repair grub, it says it did it successfully, but I still get the background image when I try to boot.

Any ideas?

(Could it be the matter that I chose UnDotum font style? That was installed in Lubuntu - but how could it be accessible in displaying boot options?)

The contents of etc/default/grub are:

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT=0
GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entries
#GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"

I have tried to modify etc/default/grub: GRUB_HIDDEN_TIMEOUT=0 to 10 GRUB_HIDDEN_TIMEOUT_QUIET=true to false and GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" to ""

but it doesn't help

Also, using Shift doesn't make the list visible.

I am looking for something like a command that would reset grub options to default.

[When trying to reinstall grub i get to this window in term: