Babbsy there can be numerous reasons for your problem. Yes some ISP's will block or filter p2p. We'd need to know your isp to know if it's a known one for this.

For the time being might I suggest 2 things: 1. Delete the LW Preferences folder & 2. Re-install Java 1.5, reboot & then try opening LW. See: Fixes for Limewire