Have you waited a few minutes ? it sometimes happens to me, and for some reasons once in a while it takes some time for the server list to even start to appear with all the servers getting added. At first, I just waited 5 or 10 seconds and switched back to the menu or restarted the game, but when I waited more, the list came back. Can’t be sure it’s the same problem though.
And put your firewall back up
Check maybe a firewall on your router if you have one, but if it worked before and the port hasn’t changed, it shouldn’t be the problem I guess.