I created a contract between myself and a client. The contract states that I will provide service A to the client. I provided service A. A third party is blocking the use of the end product of service A to be used for my client. My client no longer returns my emails or texts and has failed to respond to my invoices (sent certified mail and with return receipt).
This depends on the representations that were made at the time of forming the contract, the prior knowledge of possible interference, and the nature of the interference by the third party.
If the interference, or likely interference was known:
- To you, the vendor, and you made representations that your service would work despite this, or you did not disclose this, then you may have sold a product that was not fit for purpose and the client may be entitled to statutory relief, or may not be required to fulfil their contractual obligations
- To you and the client, and you notified them that this may prevent use of your service, and despite your advice they decided to enter into the contract, then the client is unlikely to be entitled to relief and must fulfil their contractual obligations
- To only the client, then they are unlikely to be entitled to relief and must fulfil their contractual obligations
If the interference is of a nature such that the third party was aware of your contractual relationship, caused such an interference resulting in a breach of contract, and was not entitled to create such a breach, then the third party may have committed the tort of economic interference (generally known as tortious interference) and either you or your client may be entitled to seek damages, if any.
Prima facie, you have fulfilled their side of the obligation, they must fulfil theirs and pay the bill.
@jimsug has given the reasons why they may not have this obligation, however, those are all subject to proof.
Take out a summons, state why they owe you the money and serve them. If they do have a defence or counter-claim then this forces them to put it on the table. More likely, they don't and will pay the bill.