Are you sure it's not Hibernation? That is a near-shutdown and requires a brief push on the power button to wake. Hibernation makes everything stop working.. Do not restart and don't hold down the button. Try that next time it happens and a key press doesn't work. It takes a little longer to wake and looks similar to a reboot, but it much faster than an actual restart. You can change the setting in Power Options, Advanced settings of your power scheme.