I just had the weirdest thing happen to me. I was working as usual in my vagrant box when I was suddenly kicked out. No explanation, just lost the connection. After that I was unable to log in again using vagrant ssh, as you can see below in the console output.

vagrant@vagrantbox:~/$ Connection to 127.0.0.1 closed by remote host.
Connection to 127.0.0.1 closed.
troels@host:~/workspace/drupal-vagrantbox$ vagrant ssh
ssh_exchange_identification: Connection closed by remote host
troels@host:~/workspace/drupal-vagrantbox$ vagrant status
Current machine states:

default                   not created (virtualbox)

The environment has not yet been created. Run `vagrant up` to
create the environment. If a machine is not created, only the
default provider will be shown. So if a provider is not listed,
then the machine is not created for that environment.

At first I figured it was probably just a fluke, so I did vagrant up, and Vagrant tried to build the machine again, however it already existed (i.e. I could find it in VirtualBox), so Vagrant failed creating it anew. But it still knew nothing about the box.

troels@host:~/workspace/drupal-vagrantbox$ vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Checking if box 'hashicorp/precise64' is up to date...
==> default: There was a problem while downloading the metadata for your box
==> default: to check for updates. This is not an error, since it is usually due
==> default: to temporary network problems. This is just a warning. The problem
==> default: encountered was:
==> default:
==> default: Couldn't resolve host 'vagrantcloud.com'
==> default:
==> default: If you want to check for box updates, verify your network connection
==> default: is valid and try again.
A VirtualBox machine with the name 'dev' already exists.
Please use another name or delete the machine with the existing
name, and try again.

Bugger. What to do?

Well, I tried googling a bit and I actually found a solution.

What I had to do was find the id of the vbox in the VirtualBox configuration, which was located in this file ~/VirtualBox VMs/dev/dev.vbox in the uuid for the Machine tag. I copied it and replaced the value in ~/workspace/drupal-vagrantbox/.vagrant/machines/default/virtualbox/id with the actual/new value from the .vbox file. Then I ran vagrant up again the the machine booted as usual.

Just a note: I made sure that the machine was powered off using the VirtualBox application, and then closed the VirtualBox application to make sure that there were nothing interfering with Vagrant.