[Before the problem]
Recently, I've installed Kubuntu 20.04 on my work computer with a dual boot with windows. The computer has an SSD, and a HDD. The SSD contains a EFI partition at the beginning, then the windows system partition, then my linux root partition. The HDD contain a windows "data" partition, and my linux /home.
The installation was complex, as the computer boots with UEFI. Whatever I tried, I couldn't access the grub menu; but I found out that when I activated UEFI, I booted on windows; and when I deactivated UEFI and activated legacy mode for the boot, I booted on linux (but without any passage to the grub menu).
This morning, without any changes done to Kubuntu or the boot the previous day, I booted on a dark screen without any mouse pointer. Distressed, I tried to use boot-repair with my live Kubuntu USB key. It didn't work, and it's even worst : now, when booting with UEFI, I land on a grub console ("Minimal BASH-like line editing is supported..."); and booting on legacy, I end up on a boot error (file /boot/grub/i386-pc/normal.mod not found).
Using boot-repair in every possible way didn't work; neither reinstalling the kernel, or playing with any of the advanced options in boot-repair. Nothing changes.
I was able to use the instruction in the top post of the following link to get past the grub console; but I then land on a Busy Box console where I can't get anywhere.
Now, I'm at the end of my wits, and I would really appreciate any advice to improve the situation, and at least access either windows or Kubuntu.