I'm using Windows 8 on my Dell 5520 laptop, with a problem that has been happening for the past few days I can't hibernate my Laptop. When ever I do it, sometimes it shuts down my system and sometimes it restarted to login screen.
You should check the power options first. May be there is an incompatibilities with Hybrid boot. You can access power options via the "Control Panel".
You can also boot to a Diagnostic Startup from msconfig (just search for msconfig using the search box located in the Start Menu).