If you are unable to reach any website, there may be problems with the DNS servers that you've configured. These are found in
/etc/resolv.conf and are set by NetworkManager when it connects to a network. If you manually change them, they may be overwritten the next time you connect to a network. You can edit the file to troubleshoot other nameservers, but the best way to make permanent changes to your DNS servers is to use System Settings.
Navigate to System Settings -> Network -> Wireless (or Wired). If you are connected via Wifi, select the wireless network you are connected to from the drop down. Press the "Options" button (you may have to disconnect from the network for this button to be enabled). In the IPv4 and IPv6 tabs, the method should be Automatic DHCP, meaning the network you are connecting to will provide your IP address, netmask, gateway, and DNS servers. Try entering a public DNS server (such as Google's: 126.96.36.199) in the "Additional DNS Servers" field for both the IPv4 and IPv6 tabs and reconnecting to the network.
Verify the new DNS server is in
/etc/resolv.conf and try connecting to a website again.