1st Option :
Here's how to prevent your computer from entering a power-saving state: Open Power Options by clicking the Start button , clicking Control Panel, clicking System and Security, and then clicking Power Options. On the Select a power plan page, click Change plan settings next to the selected plan
2nd Option :
To fix the problem, turn off Hibernation, and then set Standby to any value except Never in Power Management. Do this for each power scheme. Then you can turn Hibernation back on. User complaint: "On a laptop with limited RAM, generally 64 MB or less, the computer will sometimes not enter Hibernation or Standby mode.