Locks
Online migrations, snapshots and backups (vzdump) set a lock to prevent incompatible concurrent actions on the affected VMs. Sometimes you need to remove such a lock manually (e.g., after a power failure).

qm unlock

root@proxmox1:/etc/apt# qm
ERROR: no command specified
USAGE: qm [ARGS] [OPTIONS]

   qm guest cmd <vmid> <command>
   qm guest exec-status <vmid> <pid>
   qm guest passwd <vmid> <username> [OPTIONS]
   qm guest exec <vmid> [<extra-args>] [OPTIONS]

   qm clone <vmid> <newid> [OPTIONS]
   qm config <vmid> [OPTIONS]
   qm create <vmid> [OPTIONS]
   qm delsnapshot <vmid> <snapname> [OPTIONS]
   qm destroy <vmid> [OPTIONS]
   qm list  [OPTIONS]
   qm listsnapshot <vmid>
   qm migrate <vmid> <target> [OPTIONS]
   qm move_disk <vmid> <disk> <storage> [OPTIONS]
   qm pending <vmid>
   qm reset <vmid> [OPTIONS]
   qm resize <vmid> <disk> <size> [OPTIONS]
   qm resume <vmid> [OPTIONS]
   qm rollback <vmid> <snapname>
   qm sendkey <vmid> <key> [OPTIONS]
   qm set <vmid> [OPTIONS]
   qm shutdown <vmid> [OPTIONS]
   qm snapshot <vmid> <snapname> [OPTIONS]
   qm start <vmid> [OPTIONS]
   qm stop <vmid> [OPTIONS]
   qm suspend <vmid> [OPTIONS]
   qm template <vmid> [OPTIONS]
   qm unlink <vmid> --idlist <string> [OPTIONS]

   qm cleanup <vmid> <clean-shutdown> <guest-requested>
   qm importdisk <vmid> <source> <storage> [OPTIONS]
   qm importovf <vmid> <manifest> <storage> [OPTIONS]
   qm monitor <vmid>
   qm mtunnel 
   qm nbdstop <vmid>
   qm rescan  [OPTIONS]
   qm showcmd <vmid> [OPTIONS]
   qm status <vmid> [OPTIONS]
   qm terminal <vmid> [OPTIONS]
   qm unlock <vmid>
   qm vncproxy <vmid>
   qm wait <vmid> [OPTIONS]

   qm help [<extra-args>] [OPTIONS]

root@proxmox1:/etc/apt#