Better not to use the site again. Six months for investigation? That is uncalled-for. Use a better casinos. You can go to the gambling board and check the announcement thread of active casinos, use them instead.
If not that and you are sure that you have done nothing wrong, going to third parties over this matter would have been better.
If the casino is in your country, you can report the gambling site the gambling regulator.
Well, investigating for more than 6 months does seem like quite a long time. But I have seen such cases before. It probably depends on the type of investigation and who is conducting it. It may be a third party security provider so it is out of the casino's control. It does sound a bit on the longer side, though. A month or two should be the maximum in my opinion.
Not only that it is too long, it is uncalled-for and they are playing with someone's money if the person do nothing wrong. Such site should be avoided. We have better casinos on this forum other than Blackjack. There are some I have not even heard complain about before, those are the exchanges that should be used.