Check the modem (and router -if you have one)to ensure that all lights are green.
On the router you should have 4 lights.
If you only have 2 lights, your router is either disconnected-check cable connections to modem /router or has failed.
See if you borrow a router off a friend to test.
Press the Windows key & Break key together to get properties.Select Hardware. Select Device manager.
Check for any Yellow question marks, If any found, right click and select Uninstall ( they will re-install at the next boot with the correct settings.
This may cure the problem
Third- Try a new connection cable.