Hmmm... you don't say which distro you're using. Some releases of Fedora had problems with their /etc/resolv.conf files. Also, while learning ins and outs of linux it's possible to alter a setting all the while having no idea that it affects something else. If you were originally able to connect, but not now, I would check that. Also check your firewall and security settings.
Setting certain dialup modems to connect at high speeds can also create problems, although this usually only affects downloads and not page loading or name lookups.