Go to Settings which should be the second option when you pick what you want to go to on the ps3. Go all the way down til you see network settings.
Then just go through the set up of a wired connection and you should be good to go.
But if that doesnt work, just unplug ethernet cable and your modem, wait 10 seconds, plug it back in then put the cable into your ps3. Could just be an ip issue....