I upgraded my proxmox server to version 7 and was dismayed to find it had no network connections after a reboot. After much digging I was finally able to find this post which mentioned:
After installing ifupdown2 everything works fine.Sure enough, ifupdown2 was not installed anymore, and I had configured my networks with it. I had to manually assign an IP address to my node long enough to issue the command
apt install ifupdown2
Once I rebooted, everything came up like it should. Lesson learned: if you use ifupdown2, you must make sure it’s there before you reboot your server!