Recovering lost partition

I have a dual-boot Windows 10 / Kubuntu. I originally gave Windows 200Gb and Kubuntu the remaining 250Gb. Yesterady, I decided to shrink the Windows partition to 100Gb since I'm almost never using it and I need some more space on Ubuntu. So here is what I did:

• In Windows, I used the Disk & Partition manager to reduce Windows's partition.
• Then I booted on the USB key I had used to install Kubuntu
• I selected "Try Ubuntu" and launched the "KDE partition manager" (which, from what I undestand, is pretty much Gparted)
• I right-clicked on my Ubuntu partition, selected "Move/Expand". Then moved the partition to the left and expanded to the right, so it would start at the beginning of the unallocated space and end at the same place as before.
• The GUI summarized my changes and I clicked on "Apply"

And that's where the issue happened. It froze almost instantly. I could not move my mouse, the clock was not moving, the light on the USB key stopped flashing. So I waited around 10min and then decided to force shutdown the laptop.

Then I tried, to remove the USB key, and launch Ubuntu. Of course, the GRUB was only showing the rescue shell. I tried to access the boot menu and launch Ubuntu from there, but I would redirect me to the GRUB shell.

I then tried to boot from the key again. At first, I thought only the GRUB was broken, so I tried to reset it. I launched os-prober and it only found Windows. So I understood the issue was bigger.

I opened up the partition manager again. The partition was showing as moved but not extended. So I tried to move it to his original place. This time it did not freeze, and the process completed without error. Unfortunately, this changed nothing (and maybe made it worse?...).

I also tried to list partition using fdisk. A Linux partition was showing. I tried to mount it and cd into it. All I found was some broken symbolic links to "bin", "lib64" and other folders, along with the lost+found folder and a "swapfile".

There were no other partitions showing on the list, besides the Windows, EFI and recovery ones.

I also tried using TestDisk to search for lost partition. It found a long list of "Linux filesystem data" partitions that "cant' be recovered".

I know I did a lot of pretty stupid mistakes on this one (the first one being not having done a backup before touching partitions). I'm currently trying to recover my files using PhotoRec (as suggested in other posts) and it seems it's going to take 1-2 days to complete, and then I'll probably reinstall everything.

However, if someone has another idea to recover my partition, it would be really nice.

welcome to AskUbuntu. Your Windows OS is still working, right? You could download an rescatux.iso or AllinOne system rescue toolkit/AiO-SRT ( also an .iso ) and 'burn' this to a usb-stick. With Balena Etcher that will be very easy. Or use Ventoy ( install Ventoy on the stick and you can just drag and drop iso's ( yes, more than 1 ) to the stick. Link rescatux: enter link description here. Link AiO-SRT: enter link description here. Please look at their help/instructions before doing something. Boot from the stick and try to rescue your partition. And please remember that rescueing a partition can take a long time.