You wireless card is probably set to sleep on inactivity.
To reconnect, go to Start > Run, type cmd. Now type ipconfig /renew.
If that doesn't work, try ipconfig /release, then ipconfig /renew
right-click on the wireless connection, choose the power Management tab and uncheck allow the computer to turn off this device to save power. Also uncheck the second box also.
The best answers are directly in the Help and Support area. Go to Start > Help and Support and type in laptop power.