Mac no longer connecting to server

I've tried searching for my problem here and elsewhere, but no luck, so I'll just post it. I've been using my Macbook for about a year and have been able to share files with my Windows XP server with no problem. Until now. Now the server doesn't show up in my Places, and when I click Go > Connect to server, it's gone. So, I re-enter the IP address (smb://and the numbers) but just get a connection failed message. I've tried just entering the computer name, but still get the message that I cannot connect to this server because it cannot be found on the network.

It's not the server, because my son's Macbook connects just fine.

I'm sure it's something I've done, I did connect to a public wi-fi the other day, maybe something got changed. I've run through the network diagnostics to my limited ability, but no luck.

I hope someone here can guess what I've done.

Thanks.

Patti H