Happy New Year everyone!

I have planned server reboot on New Year, as I plan to restart server up-time, but my plan derailed when GRUB unable to boot

Upon searching the solution, found solution that work!

The Fix

First, you need boot into Linux Live CD, must be 64-bit, Ubuntu or Proxmox (Debug Mode) is recommended

In Live CD

After you boot, do this to scan and fix filesystem:

e2fsck -ff /dev/pve/root

Resize partition to 1GB less, this is must, because LVM ID need to change! By default Proxmox set LVM Root with 96GB, make it less 1GB become 95GB:

resize2fs /dev/pve/root 95G

Mount required:

modprobe efivarfs
mount /dev/mapper/pve-root /mnt
mount -t proc /proc /mnt/proc
mount -t sysfs /sys /mnt/sys
mount -o bind /dev /mnt/dev
mount -t devpts /pts /mnt/dev/pts

CHROOT mounted root

CHROOT the /mnt

chroot /mnt

My Proxmox is boot under UEFI and on M.2 NVMe 500GB

Common install is on SATA_1 SSD, EFI Partition is located at /dev/sda2

So, my EFI Partition for NVME Disk plugged on M.2_1 on the motherboard would be:

/dev/nvme0n1p2

Then I do this:

mount /dev/nvme0n1p2 /boot/efi

For UEFI user, do this, it will update your motherboard boot entry:

mount -t efivarfs /efivarfs /sys/firmware/efi/efivars

Then, run update GRUB:

update-grub

Install GRUB to Boot/EFI Partition:

grub-install /dev/nvme0n1p2

Exit chroot then reboot!

Source: Proxmox Forum