virtualbox.org

On my CentOS7.2 laptop, I was running VirtualBox 5.0.24, installed from yum. I have VMs running Ubuntu 14.04, 15.10, and 16.04, for various purposes. This morning yum told me of a 5.0.26 update. Before the update all the VMs were basically working fine.

After the update completed, I started up the 16.04 VM before I installed the extension pack, so it failed to start. I then brought it down and installed the extension pack, apparently successfully. When I then tried to start the 16.04 VM again, it rendered the VirtualBox menubar (I'm in full-screen mode), but otherwise the entire screen is black. It won't even display the mouse cursor when it's outside of the menubar. I let that sit that way for quite a while, and then gave up and clicked the "X" on the menubar to shut it down. Curiously, it took quite a while for the dialog box with shutdown options to appear. I chose shutdown, and the progress bar dialog appeared, but it's been stuck on 56% for about 20 minutes now, and won't advance. It won't even let me abort anything. I'll likely have to kill processes or simply reboot the laptop to make this stop.

I brought up the VirtualBox interface itself, and in the listing it shows the VM as "Powered off", but it shows "Session locked" in the tooltip.

I then tried running the 14.04 and 15.10 VMs, and they're both working fine.

What can I do here?

General advice, not specifically for your problem.
When you're describing something to a person in writing, you should ... write down the exact details. I had to read your post 3 times to figure out that "I started up the 16 VM" didn't mean "I started up 16 different VMs", or "I started up the number 16 VM", but it actually meant "I started the Ubuntu 16.04 VM". Details, I know, but they make a difference...