I had a similar issue. After upgrading to Windows 8.1, my Dell Inspiron laptop would no longer sleep. Any attempt to make it sleep would result in a full restart.
It turned out that this was due to the AMD catalyst drivers. Any attempt to remove the drivers using the Catalyst software resulted in a blue screen crash (thread_stuck_in_device_driver). In the end, I uninstalled the AMD Radeon HD 7730M device through the Device Manager, and then installed the lastest AMD drivers from the Dell website. (These are the bespoke Dell AMD mobile drivers. The latest Catalyst drivers from the AMD website don't work.) This fixed the issue and my laptop now sleeps again.