Build updates are to some extent mandatory with the Insider program -- the latest build went out to Insiders in both the fast & slow rings, so pretty much everyone updated. Then Microsoft pushed out a patch, KB3105208, which was also pretty much mandatory. Apparently some UEFI bios Secure Boot features are not compatible [implementation varies by make/model & bios version as well as what's turned on in the bios settings]. The results are not pretty -- the lucky ones may be able to turn secure boot off, but if not, from reports that copy of 10 is rendered pretty much useless.
The really lucky part is that this was confined to Insiders -- the next time it might not be. Particularly if you're running 10, with its tendency to force updates, you really, really need to be backing up regularly.
