Forwarding PCIE devices in Proxmox is pretty easy. But in case the KVM/QEMU Machine crashed the system and the onboot flag was set, it is nearly impossible to get the system back working.
Well there is a chance: Starting in recovery mode. So after the PC was turned on and GRUB comes up, select “Advanced options for Proxmox VE GNU/Linux”. Now you can select the Rescue mode. After a few seconds, it is possible to login with the root password.
If you now want to edit the /etc/pve/qemu-server/<number>.conf file, you will see the folder is empty.
Before you can edit files in /etc/pve you need to start the pve-cluster. So just type
systemctl start pve-cluster
Now you can edit your files in /etc/pve and hopefully you can unbrick Proxmox…