This is already the second time in 2-3 years that I have run into GRUB breaking on one Hetzner server. As a result, after a reboot the server becomes unreachable. You have to go to the control panel, enable booting from the rescue image in the Rescue section, connect to it over SSH, and restore the bootloader. Looking up the info every time is annoying, so in short, after connecting to the rescue system over ssh, enter this in the console:

mount /dev/md3 /mnt
chroot-prepare /mnt
chroot /mnt
mount -a
apt-get install grub2
grub-install /dev/sda
update-grub
exit
reboot now