Perhaps, like me, you work from home, and perhaps, like me, you have an evil kitten. Maybe you’ve come back to your desk to get some work done and you go to fire up your VMs and see this:
Your VM has become "inaccessible." Unfortunately, this is a critical error with VirtualBox that Vagrant can not cleanly recover from. Please open VirtualBox and clear out your inaccessible virtual machines or find a way to fix them.
Don’t panic. OK. Panic. But, after that, simply unregister and reregister your VM.
VBoxManage list vms
This will show you a list of VMs with their UUIDs.
~/Dropbox/localhost/VMs$ VBoxManage list vms
Take that UUID and use it to unregister the box
~/Dropbox/localhost/VMs$ VBoxManage unregistervm 1bba1f71-7fd1-4cd6-8375-8a37c16d643d
Then, reregister it.
~/Dropbox/localhost/VMs$ VBoxManage registervm "/Users/jennifer/VirtualBox VMs/local.dev"