People can't connect to my server

[Molecular]

OK so after alot of work I actually got the server running. It had npc's, monsters, all that kinds of stuff. Skills were working and etc etc. However, even though I could log into my server, my friends could not. Now ofcourse the first thing that struck me was: "locked ports!". Now which ports was needed to be open, I did not know, but I checked the server files and opened any port i could possibly find. Still doesn't work. So i'm wondering, what could be the problem? Does clients connect through a port not mentioned in the files? I'm behind a nat and because of that I did have to change my ip. I tried changing it back (which made me unable to connect) but my friends still could not connect.

Some help here would be reaaallly hot.