I've been unable to boot on the ubuntu 20.04 partition after my laptop shut down because it ran out of battery.

After I plugged the PC again, it booted straight into the Windows partition installed on the same NVME drive and I realised my UEFI parameters were resetted, and date and time were not correct any more. I suspect the BIOS battery, which I changed last year, might be dead again.

I changed the boot options again and got to the GRUB screen, chose "Ubuntu" and the booting process hangs as shown below:
Boot hangs there

I did try to use the "Advanced options for Ubuntu" and what I think are previous versions of the kernel and their recovery modes, with no luck.

Would you have an idea to help me get my OS back? It's my second Linux install and I worked hard to get everything working!

UPDATE 1: When starting with a liveusb, blkid and fdisk -l don't even show the disk on which the OSes are installed:
blkid and fdisk -l

However, I can see and select both OSes in the BIOS/UEFI, and can boot on the Windows partition, which should imply there's no problem with the hard drive itself:
UEFI HDD boot option

UPDATE 2:
Running blkid from BusyBox after booting fails doesn't show the sdd where the OSes are installed either...
blkid in BusyBox

UPDATE 3:
The partition on which Ubuntu is (was?) installed is visible in Windows Disk Management tool: It is the 270.45GB partition on disk 1

The question is, why isn't it found by the bootloader?

UPDATE 4:
I've just tried installing testdisk on the liveusb session and, again, the SDD with the OSes is not shown.

Thanks,
K