This may be caused by several different things, a little more information is needed. It sounds as though you have a firewall blocking your port.

Moden type and model no.
Firewalls

Are you running sp2. This has a firewall you need to configure and allow LW full access.

Some modems have built in firewalls, which need to be configured
Lord of the Rings is better of this subject.

Just out of curiosity in LimeWire under file click connect if you haven't done so already.