This happens to me on a regular basis if I leave the computer for upwards of 10 minutes. It didnt do so at first but started after a couple of days. This is possibly related to further windows updates although nothing seems to tie in obviously when looking at my update history.
I have to hold the power button in to power off. If the screens have switched off aswell they wont come back on, if they haven't I see the login picture and can move the mouse pointer but nothing happens and no combination of keyboard mashes or mouse clicks lets me see the login prompt.
In the event log (type event viewer into the start menu) under system before every Critical problem (me powering down the machine without restarting) I get distributedCOM errors talking about this guid: "The server {BF6C1E47-86EC-4194-9CE5-13C15DCB2001} did not register with DCOM within the required timeout."
I also get the same error for this 1B1F472E-3221-4826-97DB-2C2324D389AE.
This seems to be a common theme and googling these who people with the same problem
So far following the advice here http://www.eightforums.com/general-support/34141-random-freeze-issue-since-upgrading-win-8-1-a.html, and then here http://www.eightforums.com/tutorials/26512-dism-fixing-component-store-corruption-windows-8-a.html. I tried to fix my component store (whatever that is!) it did report as needing fixing and reported as fixed. I then restarted. My machine still hung later in the day so that appears to be irrelevant.
Luckily another guy at work got exactly the same machine at the same time and doesnt have the problem. We have slightly different software installed but hopefully that isnt the issue. Reading a bit more here http://social.technet.microsoft.com/Forums/windows/en-US/444a8200-8dd1-4782-898e-d7d3f5dd422b/system-freezes-on-windows-8-lock-screen and talking to him it seems likely that the fix is going to be disabling sleep in the power settings.
I'll post confirmation if I get a freeze free day today
Best Answer
I can confirm that disabling sleep has stopped the freezes. It's been over a week and I've not had any, whereas I was experiencing them at a rate of 3-4 a day