DNS server won't respond after switching Internet providers?

So my brother decided to save money and switched from charter to AT&T. Figured out that AT&T is absolute  <ADMIN NOTE: Profanity removed as per TOU> a week later and switched back to charter. My computer says its connected to the server, but my Internet won't work. It's nothing to do with charter, called a charter guy over here and he said the connection is perfectly fine. I even restored my computer...nothing, tried all sorts of settings. I'm not very tech savvy, help?!