Define "hack into"
This is a NAS, and should be on a network with some sort of firewall protection - if you open a hole in that firewall and make the NAS accessible to the public and someone who has somehow obtained your username & password gains access - does that mean it's been hacked?
You may have been hacked - but was the NAS hacked? How is the NAS to know that the person presenting the credentials (which are correct) is not authorized to have those credentials?
Now if I were to scan your ISPs network range and get a response on an open port and then gain entry without having your credentials ....
- that would be a different matter - wouldn't it?
Just change the username & password.