On my Linux Mint 17.1 machine I regularly face an error starting a VM. This error occurs after every kernel update. It says that the VirtualBox kernel driver was not loaded.
This can be solved by reinstalling the virtualbox-dkms package:
sudo apt-get --reinstall install virtualbox-dkms
And then the VirtualBox kernel drivers are loaded.
This workaround turned out fine for me, so I thought I could share it. Remark; updating the Addons can also help. Go to: File – Check for updates.