Cannot access BIOS in Ubuntu 18.04.5 LTS since kernel update

Very recently my BIOS option is not accessible during the boot.

I checked the solution of Adriaan here but I'm puzzled.

(base) user@machine:~$ grep -e "^menuentry " /boot/grub/grub.cfg
menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-####' {
menuentry 'System setup' $menuentry_id_option 'uefi-firmware' {

Well, it seems there is a UEFI Firmware Settings but seeing there are two menu entries, starting from which should I count ?

If it can help you to find a solution in the /boot/grub/grub.cfg file the first menuentry is in ### BEGIN /etc/grub.d/10_linux ### part and the second menuentry is in ### BEGIN /etc/grub.d/30_uefi-firmware ###.

#/boot/grub/grub.cfg file
### BEGIN /etc/grub.d/30_uefi-firmware ###
menuentry 'System setup' $menuentry_id_option 'uefi-firmware' {
        fwsetup
}
### END /etc/grub.d/30_uefi-firmware ###

/etc/default/grub is set as the following.

#/etc/default/grub file 
#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_TIMEOUT_STYLE=countdown
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""

Besides, looking at some boots log I suspect it's from Kernel Version update. I noticed the BIOS was not displayed those last days. Concretely in Tue Jun 8 my machine booted on 5.4.0-73-generic and since it has booted on 5.4.0-74-generic. If this is from this I am not experienced at all to do anything with this information unfortunately.

Motherboard (recognized by the OS) TUF Z390-PRO GAMING.
Version
Kernel: Linux 5.4.0-74-generic(x86_64)
Distribution: Ubuntu 18.04.5 LTS
Version: #83~18.04.1-Ubuntu SMP Tue May 11 16:01:00 UTC 2021

Counting menuentrys

Count only the highest-level instances of menuentry and submenu in /boot/grub/grub.cfg — ignore anything within a submenu. If you have only Ubuntu installed, the UEFI Firmware Setup option should be third, after the submenu for advanced options — i.e., you would set GRUB_DEFAULT=2 since GRUB counts from 0. If you’re dual-booting with Windows, the GRUB_DEFAULT should most likely be set to 3.

Viewing the Menu

I notice that GRUB_TIMEOUT_STYLE is set to countdown. From GRUB’s manual, this means your GRUB menu would be hidden for GRUB_TIMEOUT (5 seconds in your case), but with only a single number (usually at the top-left) showing the countdown.

In the ‘countdown’ case, it will show a one-line indication of the remaining time.

You probably want to set GRUB_TIMEOUT_STYLE to "menu".

Even if you don’t, you should be able to view the menu during the 5-second countdown by pressing either ESC, F4, or holding SHIFT:

If ESC or F4 are pressed, or SHIFT is held down during that time, it will display the menu and wait for input.

Getting to Firmware Setup

If your goal is to just have the UEFI Firmware Setup option show up, don’t change GRUB_DEFAULT, because you’ll enter setup each time you boot, then. Instead, try what I said above in Viewing the Menu.

N.B.: Remember to run sudo update-grub after editing /etc/default/grub.

By the way, there is one possible cause of the UEFI entry disappearing from GRUB that I have here ignored; your machine may be booting up in Legacy BIOS mode, instead of UEFI. However if /boot/grub/grub.cfg really does contain a menu entry for 'System setup' like you say it does, with the command fwsetup within it, then this is definitely not the case.

Note: the line that says 'System setup' indicates that your UEFI Firmware Setup option in the GRUB menu will instead be titled System setup. Just a heads-up. You can name this anything you like.

Hope this helps!