I can't see the GRUB Boot Manager while the disk on which Ubuntu is installed is in the first order in boot priority

I can't see the GRUB Boot Manager while the disk on which Ubuntu is installed is in the first order in boot priority, instead I see the GRUB terminal and when I enter "exit" there, my PC automatically boots Ubuntu and I can't make choice between Ubuntu and Windows. More of that, I can't see the Windows disk in Grub Customizer, so I can't configure the boot settings as I wished. What should I do to solve this ?

The grub os prober has been removed for security reasons (don't remember which reasons they mentioned exactly...). That's probably whyn you can't see windows in the grub menu. To see windows again in the menu you'll need to re-enable the os-prober, and update the grub menu.

Assuming you're at least on 22.04, open a terminal and then

1. become root

   sudo -i
   

2. configure the grub

   echo GRUB_DISABLE_OS_PROBER=false > /etc/default/grub.d/myconfig.cfg
   echo GRUB_DEFAULT=saved >>/etc/default/grub.d/myconfig.cfg
   echo GRUB_TIMEOUT_STYLE=menu >> /etc/default/grub.d/myconfig.cfg
   echo GRUB_TIMEOUT=7 >> /etc/default/grub.d/myconfig.cfg
   

3. update your grub config

   update-grub
   

   You should see something like this

   root@mypc:~# update-grub
   Sourcing file `/etc/default/grub'
   Sourcing file `/etc/default/grub.d/init-select.cfg'
   Sourcing file `/etc/default/grub.d/myconfig.cfg'
   Generating grub configuration file ...
   Found linux image: /boot/vmlinuz-5.19.0-41-generic
   Found initrd image: /boot/initrd.img-5.19.0-41-generic
   Found linux image: /boot/vmlinuz-5.19.0-40-generic
   Found initrd image: /boot/initrd.img-5.19.0-40-generic
   Found memtest86+ image: /boot/memtest86+.elf
   Found memtest86+ image: /boot/memtest86+.bin
   Warning: os-prober will be executed to detect other bootable partitions.
   Its output will be used to detect bootable binaries on them and create new boot entries.
   Found Windows 10 on /dev/sda1
   done
   

4. kill the terminal and reboot, you should see windows now.

This is a permanent setting, you'll never again have to do that.