When you get error like this

Loading, please wait…
Volume group “pve” not found
Cannot process volume group pve
Unable to find LVM volume pve/root

like this picture

just reboot then
choese  menu

Advanced options for Proxmox Virutal Enviroment

then press e

then add rootdelay=2  after pve-root ro quiet

before

after

then ctrl+x   now can boot success!

!!! remeber change /boot/grub/grub.cfg      

https://forum.proxmox.com/threads/solved-proxmox-4-pve-not-found-after-first-start.24702/