Changed DNS server, now can't connect to internet on Safari, Chrome or App Store
I always use OpenDNS when troubleshooting DNS issues, try using 208.67.222.222 and/or 208.67.220.220 as your DNS servers on your computer. Alternatively, try setting the network interface on your computer back to auto-obtaining DNS servers from your router via DHCP.

System Preferences > Network > [Network Adapter being used (WiFi or Ethernet usually)] > Advanced > DNS tab > delete any manual entries under "DNS Servers" and you should see a few entries in light grey (those are given to your computer via DHCP) Then click "apply" and you should be all set