found that in my case, when i did a sc quesry vboxdrv, it showed stopped. Trying to start it via sc.exe came up with path not found. Looking in the registry (HKLMsystemcurrentcontrolsetservicesvboxdrv) showed the path to be ??C:Program FilesOracleVirtualBoxVBoxDrv.sys, but VBoxDrv.sys does not exist there. I pointed the key to ??C:Program FilesOracleVirtualBoxdriversvboxdrvVBoxDrv.sys, and the service started fine. Not sure what happened. I also uninstalled KB3004394, but that didn't fix the issue.
I had issues for a few days now - one of my VMs got mysteriously wiped, and virtualbox was in a non-functioning state. I think my laptop rebooted because of patch tuesday, but i didn't do anything