Most of VirtualBox relies on its VMs.
But sometimes, Your VirtualBox can break. Before you attempt to fix it, (or your trying to get a new virtualbox version)
Lets make sure your VMs are safe so that you don't have to spend your stress reinstalling them,
First, Open the VirtualBox VMs Folder. (it is mainly found in your personal folder and is not hidden.)
Then go into each of your VM Folders and take out the .vhd or .vdi files (in my case its .vhd) and put them on your desktop.
Now, make a Folder on your desktop and put the VHDs or VDIs in them.
Now it is safe to reinstall virtualbox or do whatever you need to do w/ it.
Once you have done that, Remake ALL of your vms, and This time, Don't create the Hard disk. Just use the one you put in your folder on the desktop that corresponds the OS you want.
When you startup your VMs, They will have an OS w/ all your saved VM files on them.
That's how to do it!
Thanks and Hope this helped!
--MiloTheDev.