My phone literally just started doing this today. It locked up a couple times and then the "soft reset" itself wouldn't complete...it would lock/blank screen even during the reboot part after I initiated the soft reset.
I uninstalled a couple apps which were updated recently to see if that would help, but nothing. So I suspect something is running in the background and draining the battery, but there is nothing to tell me what that background task is. I don't think it's wifi, wifi sense, or any of those features. I've had those on for several weeks with no issue.
I would try a hard reset, but of course I can't backup my phone because it keeps failing, which seems to be a global problem with 8.1. Not really interested in doing a hard reset, unless I move all my apps and music to my SD card so I don't have to redownload everything.