GRUB maybe not correctly installed (respectively does not find file /boot/grub/grub.cfg). Try the following:
- Boot with a Linux live Media (perferrably Debian or Ubuntu)
- investigate the Proxmox boot disk - check if /boot/grub/grub.cfg is ok (probably yes; if no you should better reinstall Proxmox)
- reinstall GRUB at System bootdisk with the proper /boot/grub/grub.cfg