It sometimes happens that when I shutdown Proxmox for some hours, that after restart, the network interface does not work. I figured that Proxmox is rotating the network interface numbers which leads to the Linux network bridges to not map to the correct network interface and thus leads to no network connectivity. The only thing that helps is to adapt the /etc/network/interfaces file.

Some commands that can help:

brctl show
ifconfig vmbr0
dmesg
lspci | grep -i ethernet