If you have a high speed modem, either DSL or Cable, shut it off for at least 10 minutes and shut down the computer. Make sure that the line connected to the outside world is solidly connected. Sometimes the cables at the back of the router and cable box need reseating.
Boot up in safe mode with network connections and try it of course after you turn on the router and modem box. You can reinstall the network adaptors.
Another way, if you have Sysem restore operating is to restore back to a period that things were working and that should clear things up if it is a software issue