On some servers it goes blank when attaching the KVM from Hetzner when trying to debug issues, one solution that has helped me in the past is editing the GRUB configuration with the following workaround.
Please be aware that making changes to GRUB may prevent your system from booting correctly, and you might have to boot into rescue mode to undo the changes if you experience problems.
Workaround for Debian
Edit the grub configuration as root
nano /etc/default/grub
Comment out the original line and add this instead
GRUB_CMDLINE_LINUX_DEFAULT="nomodeset xdriver=vesa brokenmodules=ast"
Now update grub to persist the new change
update-grub