Here is a clip from the FAQ at the head of this section-

3-6Q. I was connected with LimeWire just fine, but now it doesn't connect. If I click File > Connect, nothing happens.
3-6A. Try typing these host cache addresses into the address field and clicking the "Add" button in the options menu.
connect1.gnutellanet.com:6346
connect2.gnutellanet.com:6346
connect3.gnutellanet.com:6346
connect4.gnutellanet.com:6346

Beyond that your ISP may now be blocking traffic on port 6346 - If that is the case you might want to try using Telnet to test your ability to connect to Gnutellanet. You can test this by opening
Start/Run and entering:
telnet 64.61.25.171 6346

A window should open with 'Gnutella OK ' in it ... if not then you are most likely being blocked somewhere down the line with the most likely culprit being your ISP