I have used the site for much larger amount of money and never had any problem.
Would be happy to hear more info , it said that you account is locked? It doesn't show any reason for the lock?
Or you just can't login to the site and you concluded that your account has been locked?
Try to reach them out via the email above, and if you couldn't, I would recommend you to message user jakem who claims to be 999dice owners:
https://bitcointalk.org/index.php?action=profile;u=185799I doubt if he will answer from that account, but you can give it a try.
I think that you shouldn't be worried as 999 dice has proved to be legit, they have been here for years.
When you played on very huge btc last time, may be the price noy even hit $5,000 so it does not matter to them, someone already claimed really huge amount on their site but in the end they got nothing as their payout. 999dice sure is a scam site yet people still played it here and now you said it legit? This is really absurd thing to say here
I guarantee that if you already ask their owner, they still do not make the payout as well