if you are behind a router then it could be something as simple as your LAN IP Address changing, which would mean having to change the port forwarding to the new IP Address it has changed to, check that your LAN IP Address has changed (if that is the case then setting a static one might be a good idea to stop it happening again in future) if not then there are a few other things to check, but that is the most common problem people have so best to start there...