Did you happen to share the key with anyone else? your key gets auto revoked if its detected to be used on two far away ip's within a short timeframe, on client instances of BL (dedicated servers are exempt)
Example: someone opens blockland the game on a computer in Virginia, and another on a computer in California, and both auth within 5 minutes of each other, probably more than a few times so its established definitively that two computers are using the same key to play bl at the same time