If you are working on upwork, once you've completed the desired job you should have some of the evidences like contract details, interviews or message screen-shots that is required to prove that you are legit. Having all in your hand, you should refer to upwork report your client section.
From there you will be able to select your
question category in your case select
work, feedback, and disputes. Then you have to select another option,
Unresponsive Client will be best suitable in your case. Having that selected, attach your evidences in a zipped folder and fill other relevant informations (email, description about your problem) and submit.
Note that, by any chance, if you've contacted with the client outside of the upwork network about the contract or payment, upwork authority will not be liable for this scenario or even you could've been get blocked instead because of violating upwork freelancer rules. But if you haven't done that, in most cases, upwork solves these type of issues within one or two weeks.
In case if you are not working on upwork, most freelancing websites like freelancer.com, peoplesperhour.com or fiverr.com has a support section from there you will be able to submit a query or open a support ticket. You just need to follow the same rules as showing them the screen-shots and relevant files and information to get paid.