If the Proxmox OS was not mirrored or if the OS is beyond repair, it may be necessary to reinstall Proxmox on a new or reformatted OS drive. If the node was part of a cluster, then after the OS reinstall, simply re-add the node into the cluster. If both IP address and hostname are the same as before, it may be necessary to add the node forcefully with the -f option in the pvecm add command. Reinstalling Proxmox and then re-adding to the cluster may be a faster solution in most cases than trying to fix an OS-related issue, but each use case will vary based on the environment and where disk images are stored.