If you did not make any changes to your SME server which may have caused this problem, there can only be two causes left:
1. There is a router or firewall in front of your SME server which is not forwarding port 80 to your server, or:
2. Your ISP is blocking access to port 80, because they don't want you to run local servers. Verify this with your ISP. I also noticed that port 25 (SMTP) does not give any response. Is this intentional or (also) the effect of your ISP's policy?