if you were testing cloud-init - something went very wrong. I had to reinstall Proxmox from the scratch after doing this
you may have accidentally given the same IP to the guest OS as the one used by Proxmox. I'd try logging into the guest OS and shut it down and hope that Proxmox becomes available again. Alternatively, you can change the IP of the guest OS and reboot the server.
This is tricky. What you can try is blacklisting the mac of the VM within the router settings and reboot the router. In theory, if there is IP conflict and one of the devices is blacklisted, the other one should get the IP. You will regain access to host and from there you can try changing the VM IP address.
I looked closer to the screenshots provided and this seems indeed related to cloud-init.
Unfortunately i had the same issue once and the only thing i could do is reinstall Proxmox and import the VMs from backup.
Sorry i cannot help you with this...
It happend to me some times ago, and your are able to solve this without reinstall. Try to remove cloud-init from the Hostnode itself, then change or check the hosts, the interfaces and resolv.conf. Now reboot and check your system.
I recommend you to read the wiki article carefully, i didn't do this too