I have launched the command:
sudo grub-mkconfig -o /boot/grub/grub.cfg
Now the Grub menu at boot has disappeared and the computer boots automatically into linux.
How could i recover the possibility to select kernel and os at boot?
Thank you
You have likely disabled the menu in /etc/default/grub, maybe following this guide Boot Manager Configuration | CachyOS. You can reverse the related parameters to get the boot menu back.