From my experience this normally occurs as a result of the hard drive stopping. Sometimes the hard drive is set to turn off even before the PC goes to sleep. Go to "Control Panel", then "Power Options", then "Change advanced power settings", then "Hard disk", then "turn off hard disk after" then On battery, Plugged in and change both to "Never".
Putting a pc to sleep keeps power on the motherboard and I have had to replace a few laptop motherboards because the laptop was put to sleep. If you have enough free space on the hard drive I recommend using hibernate.