Assuming you gamble on a mid to high-rated gambling website and you won big but they refuse to pay up, what and what are the legal way to make sure that the platform pays up your money?
Assuming that you have not done anything against their ToS or violated their rules, one of the best things to do is to contact support and ask for more information regarding the matter at hand. Also, while waiting for their response, I suggest that you gather supporting evidence and proof about your account such as screenshots of what happened and how you are not able to withdraw your funds.
If in any case, they won't answer your question or have not provided any clear information, I suggest that you contact them here on the forum and expose your issue to the public. This way, you'll be able to get their attention as they'll be risking their reputation if they will not be able to answer and provide any clarification on your issue.