This situation is faced by many freelancers, there may be many constraints, problems that results this situation faced by freelancer. In this problem try to relax and don`t panic, if you took the wrong step now then you may suffer financial loss or you loose reputation.

1) try to realize the client that although 100% work is not completed but due to some specific reason i was not able to make 100%
2) ask the client to hold the payment, let me give one more chance to improve
3) If the work you were doing is a product, that may benefit you on some other markets, platforms then try to sell that there, with client`s permission. if the situation is worst then try sayig good bye to client in good words and have your share from the other market.
4) To avoid this situation, accept the work only that you are able to meet in a specific time, committed to client.