No websites will load

Check your network interface (Wi-Fi or Ethernet) in the Network pane. It should say the connection is active and has an IP address. If not, reconnect (see above). If all looks well here, check your service provider’s status page for a known fault.

Try other domain name servers. In the Network pane, click Advanced > DNS, note the two addresses on the left (your provider’s DNS), then use the minus button to remove them. Add Google’s (bit.ly/dnsgo) or OpenDNS’s (bit.ly/dnsop) servers instead.