My 15" Surface Pro 2 has the exact same issue and it only started recently. It must be due to a recent update.... When I close the screen (flipping down the tablet onto the keyboard section) the tablet will enter sleep mode and it seems like it will always wake if done within an hour or so. After a few hours the PERMA SLEEP has engaged and I need to hold the power button for 20 seconds for a hard reset (I haven't tried the detach technique that Jon talks about).
My guess is that many are affected by this.