Hi,
Boxcryptor accounts do not suddenly "disappear". I assume that you are trying to use a wrong email address or password.
Even as a free user you may write us in support, we simply cannot guarantee a timely answer as paying customers are served first. If you send us one encrypted file (with .bc file extension) to support@boxcryptor.com, we can look up the email address of the corresponding Boxcryptor account.
Best regards,
Robert