Fresh Proxmox VE 2.1 installation with defaults can't be reached or pinged
Solution 1:
I met this problem and eventually solved it. It's the problem of the LAN we are in. For some reason my LAN doesn't allow me to manually assign IP address when there's a DHCP server out there. The Proxmox installation program use DHCP to detect the network settings and then try to assign the IP manually(by OS) without using DHCP again after reboot. The solution is to ask the system to perform DHCP again by following commands:
dhclient -r vmbr0
dhclient vmbr0
Then the problem should be solved.