VM has become 'inaccessible' - Vagrant no longer working

this works for me:

In my "C:\Users{user}\VirtualBox VMs{vm-id}" folder are two files

• {vm-id}.vbox-prev
• {vm-id}.vbox-tmp

Renaming from "{vm-id}.vbox-tmp" to "{vm-id}.vbox" solved my problem and i can call "vagrant up"

You can simply delete the .vagrant folder from your project folder and run vagrant up again.

This worked for me

After some digging through the debug output, I discovered that even though the actual VM is intact (I can load and run it from the VirtualBox GUI app), somewhere in its guts, VirtualBox flagged this VM as "". Vagrant, rightly believing what it's told, spits out the error message.

After looking at VBoxManage's help, I found that one its commands, list vms, unsurprisingly lists all of the VMs registered with VirtualBox:

$ /cygdrive/c/Program\ Files/Oracle/VirtualBox/VBoxManage.exe list vms
"precise64" {3613de48-6295-4a91-81fd-36e936beda4b}
"<inaccessible>" {2568227e-e73d-4056-978e-9ae8596493d9}
"<inaccessible>" {0fb42965-61cb-4388-89c4-de572d4ea7fc}
"<inaccessible>" {c65b1456-5771-4617-a6fb-869dffebeddd}
"<inaccessible>" {9709d3d5-ce4d-42b9-ad5e-07726823fd02}

One of those VMs flagged as inaccessible is my lost VM! Time to fix VBoxManage's wagon, by unregistering the VM as inaccessible, then re-registering it with the correct name:

1. Open the configuration file for your lost VM. Mine was saved to C:\cygwin\home\Philip\VirtualBox VMs\rails-vm-v2\rails-vm-v2.vbox
2. Find and copy the value of the uuid attribute of the Machine node. Mine was 9709d3d5-ce4d-42b9-ad5e-07726823fd02.

3. In a Windows command prompt (or Cygwin terminal), unregister the VM with the unregistervm command, using the [uuid] value from step 2:

   $ C:\Program Files\Oracle\VirtualBox\VBoxManage.exe unregistervm [uuid]
   

4. Now register the VM using the registervm command, with the path to the VM configuration file:

   $ C:\Program Files\Oracle\VirtualBox\VBoxManage.exe registervm C:\cygwin\home\Philip\Virtual VMs\rails-vm-v2\rails-vm-v2.vbox
   

   Now you should be able to start the VM as expected.

Source : http://www.psteiner.com/2013/04/vagrant-how-to-fix-vm-inaccessible-error.html