I have also had this problem with the latest Fast version of 15031. After spending a day and nearly two with Antivirus programs etc. I decided this morning (without looking at this forum) that it might be WindowBlinds 10 causing the problem, as it was the last thing I used before the screen going black. I also decided to reset Windows 10, but the last time this happened - about a couple of weeks ago - I did this and spent nearly a week getting everything back on: so I was a bit loath to go down this route again.
However after 5 minutes this morning I found that stopping the WindowBlinds service in Task Manager by using [Ctl][Alt][Del] - either Stop or End Task - I could again use [Ctl][Alt][Del] and Sign Out. If I signed in straight away my Desktop can back again. At this point I could I could again use [Ctl][Alt][Del] (Left click from Start) and remove WindowBlinds 10. After a Restart all was well - apart from WindowBlinds 10 not running.
Note! Downloading and trying to install either the latest version or the beta version of WindowBlinds 10 crashed the Desktop. I will also look at this forum first if anything like this happens again! Deskscapes 8 seems to work ok but Object Desktop Manager seems to crash whilst trying to install WindowBlinds 10.
I have been using WindowsBlinds 10 on all versions of Windows 10 Insider Previews but It is only recently that I have had this problem.
I hope this helps any other Insider Previewers!!!