Kernel Driver for VirtualBox VMs not installed (Ubuntu 12.04)
When you upgrade the kernel on Ubuntu 12.04 for some reason it does not necessarily upgrade the kernel modules required by VirtualBox to run VMs. To solve this run the fllowing two commands:
sudo dpkg-reconfigure virtualbox-dkms sudo modprobe vboxdrv sudo modprobe vboxnetadp sudo modprobe vboxnetflt VBoxManage hostonlyif create