Try deleting your gnutella.net file... see this thread for more info: If LW Will Not Connect

Also this link, which has slightly different instructions for finding the gnutella.net file in Vista: internet connection missing? - LimeWire Forums

Otherwise, try the other fixes in the first help thread - start off by deleting LW's preferences folder. If you're still stuck, please post back with your system details as shown in the help thread.