Do you have backups of the vms outside of your cluster?
Then it's propably better to reinstall everything with the newest version of proxmox before restoring the backups (this can be tested in a vm before!).
In any case (even when you mange to fix the current failure in your environment) you should update to a newer version since you won't get security upgrades for 5.2.1.
Here is a link to a recent disccusion how to approach such an update
https://forum.proxmox.com/threads/very-out-of-date-cluster.156606/post-715805