Could someone help me re-enable multi-site?

I need to re-enable multi-site admin to my install. I was working through a domain change and the site wouldn't come up. I called my hosting plan's tech support and they told me there was a circular re-direct that was causing the issue. They had me delete the information on the config.php that referred to multi-site. It was 2 a.m. and I just followed along with their instructions. Now I don't know enough to re-enable multi-site to my main domain.

Thanks for your help.