Here are two solution you can Try and I hope At-least one should work for you.Solution 1)It sounds like your laptop may be going into Hibernate or Sleep mode as part of its Power Management scheme.
You can change the Hibernate and Sleep Settings in the Power Options:
To do this:
Place your mouse over the battery icon on the taskbar and right-click.
Choose "Power Options".
(You can also access the power options through the control panel:
Start>Control Panel>System and Maintenance>Power Options.)
Under the plan being used on your computer, click on "Change Plan Settings".
Change the presented Settings as desired. If the settings you wish
to change are not shown on this screen, proceed to the next step.
Click on the "Change Advanced Power Settings" link (near the bottom of the Window).
Scroll down the list to the "Sleep" entry and Expand it. Expand the
sub-entries and change the settings as desired. Look at the rest of the
settings on the list as make changes as desired. Click "Ok".
Click "Save".
SOLUTION 2) In the power setting there is a screen brightness bar. When on power set
it all the way to the right on AC power and set it to the middle for
battery power.IF both the above will not work then I'll Suggest you to show your laptop to some local repairing center.