more info needed…
That looks like Proxmox ? If it’s not proxmox, what are you running and which version, along with disks and how you assign them.
- at the grub prompt, can we have just what is returned from;
ls
- then for each partition, a screen of what is returned - will help locating where your root partition has gone, trailing slash needed…
ls (hd0,gpt2)/
ls (hd0,gpt1)/
#edit - sorry looking at your screen shots 
so hd0,gpt2 = sda2 so try;
set root=(hd0,gpt2)
linux /boot/vmlinuz-4.19.0-21-amd64 root=/dev/sda2
initrd /boot/initrd.img-4.19.0-21-amd64
boot
if that boots then run
sudo update-grub