I've had my KDE Slimbook running without problems for several years.

Earlier this month it stopped booting, I looked at the BIOS and noticed that the date was wrong, so I assumed that the internal CMOS battery was empty and I changed it.

Not, the BIOS shows the correct date, but I still cannot boot.

I don't remember how it was installed (with/without secure boot enabled). I've tried the different options but I doesn't boot.

If I try to boot with secure boot disabled:

Reboot and Select proper Boot device

or Insert Boot Media in selected Boot device and press a key

also, if I select in Boot Override "Launch EFI Shell from filesystem device" it says "Not Found"

If I try to boot with secure boot enabled:

It enters directly in to the BIOS setup utility.

However in Boot, is shows Boot Option #1 as [Ubuntu (Samsung SSD ...] but i select it in Boot Override, it directly enters the BIOS again.

The same happens if I select Boot menu with F10, I can choose ubuntu, but it boots into the BIOS.

With a live CD I can access to my data, and the partitions are as follows:

/dev/nvme0n1p1 -> fat32 -> 512MiB

/dev/nvme0n1p2 -> ext4 -> 457.34GiB

/dev/nvme0n1p3 -> swap

I thought that the fat32 partition stored the efi boot information, but I see it empty from the file manager (partition manager says that 9.70 MiB are  used)

The BIOS Version is 305_SLIM (12/20/2017)

How can I boot into my system?