Virtualbox VM (spawned by Vagrant) running but inaccessible. What now?

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http://serverfault.com – I have a Virtualbox VM running Ubuntu that was started by Vagrant. At some point my ssh session connected to the guest stopped responding. I tried "vagrant halt" from a terminal window on the host (OS X). The shutdown process seemed to also hang. Shutting down the Oracle VM VirtualBox Manager doesn't shut down the VMs themselves. Is there a way in either Vagrant or VirtualBox to force the running VM to shutdown? When running desktop guest OSes, closing the GUI window presents several options for shutting down the guest, but I don't know how to do the equivalent when the guest is running headl (HowTos)