I had the same problem a couple of months ago. I found a solution in another post that suggested that I should download a new gnutella.net file from some place (I don't remember where). That fixed the problem then.

Today I had the same problem again. This time I just deleted the gnutella.net file (after completely closing LimeWire). That also solved the problem.

My son and I had both the same problems under different user accounts on the same PC. For some reason, on my user account the file was located under C:\Users\<Username>\.limewire. On my sons user account it was located under C:\Users\<Username>\AppData\Roaming\LimeWire.

On my user account, the gnutella.net file was recreated automaticly after I restarted LimeWire and the connection worked again. On my sons user account I had the same problem again the first time I tried. I deleted the file again and restarted LimeWire. This time the file was not recreated, but the connection worked.