Are you tired of turning your computer off and on again, after you let it sit for a while, unused, just to restore network connectivity?
Check Power Management settings, in the Network Adapter Settings wizard, which is accessible from the Connection Properties wizard.
On the Power Management tab, look at "Allow the computer to turn this device off to save power". Is that checked? If so, uncheck it.
If the above solution doesn't work, or isn't applicable, diagnose the problem a bit. When this happens again, open a command window, and type two commands, one after the other:
Observing whether or not this helps can be a useful step in diagnosing the problem.