Thanks for the flag @zabadabadoo!
We've investigated this issue and have found that specifically the upstream proprietary NVIDIA 545 driver package currently doesn't have a kernel module that's compatible with Linux 6.8 yet. We've found that most other versions of the NVIDIA driver package should nevertheless be compatible with the latest kernel update:
550 
545 
535
530 (auto-upgrades to version 535)
525 (auto-upgrades to version 535)
520 (auto-upgrades to version 535)
515 (auto-upgrades to version 535)
510 (auto-upgrades to version 535)
495 (auto-upgrades to version 535)
470
465 (auto-upgrades to version 470)
460 (auto-upgrades to version 470)
455 (auto-upgrades to version 470)
450 (auto-upgrades to version 470)
440 (auto-upgrades to version 470)
435 (auto-upgrades to version 470)
430 (auto-upgrades to version 470)
418 (auto-upgrades to version 470)
390 (legacy driver)
If you currently have the NVIDIA 545 drivers installed, you can resolve this issue by temporarily booting into the older 6.5 kernel using the Advanced Options in the bootloader menu. Afterwards, select the NVIDIA 550 drivers (the latest stable & recommended branch) through the Additional Drivers utility: Activate NVIDIA Graphics Card - Zorin Help
Alternatively, you should be able to open the Terminal (Ctrl+Alt+T) and enter these 2 commands:
sudo apt remove nvidia* libnvidia*
sudo apt install --install-recommends nvidia-driver-550
Afterwards, restart your computer to apply the changes.