More than likely this is an issue with your firewalls (third party and built-in to your OS) and networking settings. Also, your router and modem probably have firewalls or packet routing systems in place that make it even less likely that traffic will reach your computer correctly. Unless you already know how to resolve these issues or can find a good tutorial on your own, I wouldn't suggest running a server.
I try to be helpful on here when I can, but I can only respond to something specific or with some kind of information I can work off of. "Server is not responding." only indicates one thing: Their computers can't see yours. This is a good thing if you're trying to keep out those mean, evil hackers, but a bad thing if you want to run a server of any kind.