Hello,
I was updating the firmware of the HP desktop under Windows11 (as per usual recommendations from MS), then shut down the PC, went away for 1 week, then ẃhen trying to boot again, it dont start:
- BIOS updated to latest version
- Linux HDD 500GB where the Debian11 OS and most important data are, dont boot.. or it say no OS detected when I try to start the PC
- windows11 dont boot on the separate MS SSD
First: no panic, the desktop start a lubuntu live USB (the debian live did not start due to the AMD graphic card in it) and all data of the HDD can be seen = no data loss so far

Now, I would like to find a solution for making it working again in any way:

- could we have an UEFI setup which messed up during the BIOS update? (reason why the OS on the HDD is nomore recognized). So now, how to update the UEFI (new version?) on the HDD for the BIOS to (perhaps) start the Linux on the HDD again? with linux tools (since I cannot start windows11, I cannot make anything with it).. perhaps the use of rEFInd could help?

- an overcurrent during the one no-use week could have destroyed the SSD? (the SSD drive is NOT seen by the Lubuntu live which is weird; except the Lubuntu live cannot see Windows partitions). Any hint why this happens is welcome.

I am further searching a solution in the net..
- Windows11 and UEFI seems to be dramatic together..
- Lubuntu should see the windows partitions.. or not..
- till now, I was happy approx 3 years with my double Win/Linux boot. Snif.

Comments/remarks are welcome.

BIOS upgrade may well have cleared NVRAM so that no boot entries remain.

You should fix Windows first, so that it doesn't unfix whatever fixing is required for Debian. Get out your Windows installation media and boot it into repair installed system. If you can't find it, download and burn a fresh .iso.

After Windows is booting normally you should be able to "boot installed OS" by booting Debian 11 installation media, then do anything you would do on a normal Debian 11 boot. Alternatively, boot any live Linux media, and use efibootmgr to create an entry for booting Debian 11.

One very common reason for that is leaving windows in a hibernated state so Linux won't mount the filesystems and they won't be accessible. You have multiple drives? Does windows boot when you set that drive to first boot priority in the BIOS firmware and these are both EFI installs, correct?

To 1. Yes. One (original) SSD for windows and one HDD for Linux
To 2. My BIOS dont allow a priority boot. It had previously booted into the Linux HDD which offered the choice to further boot on that HDD or to go into the Windows SSD
Now, I disconnected the linux HDD for solving the general windows issue and will come back here when it is solved (I contacted HP; the computer is going into a boot loop with windows only).

Lots of speculation, too little hard data.
Stick the HDD back in and get boot-repair if not already on the Lubuntu liveUSB. Run the report creation option (don't run the repair itself at this stage). Post the report here in [code] tags so we can read it.

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Re-install of the Win11 was done on the SSD and the PC boot again into the Win11 SSD.
Boot parameter changed in BIOS:
1) activate legacy boot and disable UEFI: no boot at all in SSD/HDD
2) desactivate legacy boot (activate or deactive UEFI boot): boot only into Win11 (the HDD grub with win11/debian11 choice menue dont appear).
Now I will have a look at the #5: it should make 1) working.

Update1:
- I putted SuperGrub2Disk into a Ventoy USB Stick
- enabling legacy in the BIOS made the USB Stick booting
- then choosing SuperGrub2Disk
- searching for the installed systems (linux entry found)
- then "return" on that entry
- the Debian11 boot
Summary: the BIOS found the Ventoy USB (then Debian11 via Grub2Disk) but the BIOS could not see the HDD with grub entries on it.

Update2:
- when booted in Debian11 (see update1), under ROOT, use the command grub-install
- reboot
.. it start again into the grub menue of the HDD where we see Debian11 and Windows bootmanager
- Debian11 boot now perfectly when coosing Debian in the grub menue
- when choosing the windows menue line, the error appear "error: file '/efi/Microsoft/Boot/bootmgfw.efi' not found."

So, we are nearly there: the brub menue list was probably not updated to allow the windows boot. I will now have a look at this (for me, thats a minor issue; plan B is to start the PC with F9, then choose the windows boot).