Sounds to me like a router issue. Routers sometimes will reissue your "license" for a certain internal IP address. With multiple computers on the same router, your forwarded ports may no longer be forwarded to YOUR computer, but to another on the network. Check your router settings and make sure it's forwarding ports to YOUR computer's internal IP address. As a note, setting your computer's internal IP up as static will prevent this.
If you don't have a router, then it could be various other issues and I don't really know what to tell you to help.
