You may encounter the following condition, particularly when operating on a crashed or otherwise peculiarly paused VM:

$ VBoxManage startvm vmname VBoxManage: error: The machine 'vmname' is already locked by a session (or being locked or unlocked) VBoxManage: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component MachineWrap, interface IMachine, callee nsISupports VBoxManage: error: Context: "LaunchVMProcess(a->session, sessionType.raw(), ComSafeArrayAsInParam(aBstrEnv), progress.asOutParam())" at line 727 of file VBoxManageMisc.cpp

It can be remedied by running:
$ VBoxManage startvm vmname --type emergencystop

Then start the VM per normal circumstances:
$ VBoxManage startvm vmname