Windows 10 installed a major update automatically. In this case it was the anniversary update from a year ago.
After updating, it got stuck during boot on a spinning circle.
Very simple fix – shutdown all VMs, then update virtual box & extension pack. After doing so, shared folders did not work any more. This was solved by updating virtual box guest additions. It’s hard to find an iso…. the trick is to navigate the poorly publicised download server: http://download.virtualbox.org/virtualbox/