Make sure that LW's being allowed through the Windows firewall.

What version of Java are you using?? In LW click on Help > about limewire & you'll see. If your Java version is 1.6.0_01 or 1.6.0_02, try this version instead. Download the JRE offline installer & remove any other Java versions before you install this one.
Archive: Download Java 2 Platform Standard Edition (J2SE) 5.0 Update 11

Try deleting LW's preferences folder when LW's properly closed. See point 3 here & also point 2 if you've used older versions of LW in the past (there could be a folder in either location).
Fixes for LimeWire
This will kill all of your settings within options, you'll have to go back through afterwards & reset things. If you're not sure of your settings, go through LW's tools > options & write them all down, so that you can reset things later.

When LW was connecting, did you see the firewall icon? Your modem-router has a firewall. If the firewall icon was always showing, try port forwarding, the instructions are here.
Port Forwarding for the Netcomm NB5Plus4
Don't forget to set up a static IP address first. LW's listening port, port in the 'manual port forward' box & port forwarded within the modem should all be the same number.