The message will be stored on the servers. It's not readable by anyone that might access it, in case you wonder what happens with it. Viber messages use latest state-of-the-art encryption protocols. No one is able to decrypt them, because the encryption keys are generated in a very secure way. When you activate your account again, you should receive the message, but the server might have a policy in effect which will delete the message if it's not answered in a timely frame.