I've had this festering problem 3 times in the last month with different machines. The way that has worked for me is to force a shutdown. If it will then start normally fine - if not, force a shutdown about three times until it boots into advanced startup options. Work your way through the menus until you can System Restore. Once starting normally, go to Update and change the setting so that you are alerted to update, but don't download automatically. There will be upwards of a hundred updates to do before you can install 8.1. Untick them all, then from the top select about 20. Download and install, then continue in similar fashion until updates are complete.
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