Hi Axebros! First:
* ***Do Not Power On***: If you haven't already, do not turn on the camera. This can prevent further damage.
* ***Check for Recovery Options***: Some cameras have a recovery mode. Check the Nikon website or user manual for your D3000 to see if there's a way to restore the original firmware.
Then you can:
* ***Try to Enter Recovery Mode***:
** Remove the battery and memory card.
** Reinsert the battery (without the memory card) and hold down the ***Menu*** button while powering on the camera. If it enters a recovery mode, follow any on-screen instructions.
* ***Reflash the Correct Firmware***:
** Download the correct firmware for the D3000 from the Nikon support website.
** Format a memory card on your computer (FAT32).
** Transfer the correct firmware file to the root directory of the memory card.
** Insert the memory card into the D3000.
** Power on the camera while holding the ***Menu*** button to see if it recognizes the firmware.
Hope this helps Axebros! Happy camera fixing.
(Formatted for easy reading by ChatGPT 4o. I never write my answer using Ai)