You should thank your lucky star, it could have been much, much worse. Whoever used your account did not delete your characters. Stuff is just stuff, but a character is irreplacable.
Another thing is that the GW username/password can be the safest in the world, and it matters not if your PlayNC account is unsafe. Especially their stupid birthday and "secret question" stuff are basically made to be reverse engineered. And to top it off, afaik, you cannot easily change those data if you think they have been compromised. So if I were you I would have a closer look at my PlayNC account, and think hard if anyone could have gained access to that.