Unfortunately, unless you can find the dongle with cubase 6 on it you are pretty much out of luck. The licence is the actual dongle itself and there is no way to get it back if you lost it or it was stolen.
If the dongle ever breaks though that's a different story I have heard, but yeah in this case unfortunately you are out of luck.
If the dongle ever breaks though that's a different story I have heard, but yeah in this case unfortunately you are out of luck.