Proxmox is a very good KVM-based virtualization system, but sometimes small problems arise.
I had a case where a backup crashed during the night, and it was then impossible to restart the VM, or even to modify its configuration.
There is a command that allows you to unlock the VM without restarting the hypervisor.
To do this, you need to connect via SSH or the GUI shell, and run the following command:
qm unlock VMIDReplace VMID with the VM's identification number
Once running, the virtual machine is manageable again as before.
This trick also works on containers.
At the time of writing, it is not possible to unlock VMs and containers from the GUI. However, this feature may arrive in the future.
