Lots of people are finding that they can't connect because there's a problem with their gnutella.net file...when they download a new version of LW that file doesn't contain any host IPs to connect to. Other people might find that their preferences folder has become corrupted somehow or that their Java's too old/corrupted, or that their modem/router needs to be configured/reconfigured, or that their ISP has started blocking ports, or that their software firewall needs to be configured/reconfigured for some reason...could be so many things!

Try turning off the AT&T firewall, turn on the Windows firewall & configure it to allow LW (see link below)...does that change anything?
User Cant Connect - LimeWire

Go & find the gnutella.net file within LW's preferences folder...how big is that file?

Uninstall LW (start > all programs > LW > use the uninstall option). Now use your computer's search to look for all files & folders with limewire in the name (make sure the search is looking within hidden files & folders, & that you're not looking for case sensitive names). Delete what you find.
If you've changed LW's saved folder from the default location, & are saving files to a folder with limewire in the name...careful that you don't delete that saved folder & lose your downloads!

Restart your computer. Now reinstall LW & make sure to use the offline installer (when you click to download, cancel the dialog box which pops up & choose the option to download the offline installer). Start LW, go through tools > options & see whether you can change the firewall config settings now.

If LW still won't connect after the reinstall, shut LW down properly & then go back into LW's preferences folder & replace your gnutella.net file with one from the link in the thread below.
If LW Will Not Connect

Restart your computer, start LW.

If your router's pages don't look like the ones on the portforward site, maybe go through the router's manual & see if there are port forwarding instructions there.