Why these disputes are hard
Because there is usually no contract, only a conversation. Or a one-page contract stating the price and nothing else.
The argument is then about what was agreed, and proof is difficult for both sides. Which is why anything recorded in writing, a message included, carries more weight than people expect.
What to do now
Stop paying anything further before you have established what has actually been done.
Document the state of things: dated photographs and video of every room, including hidden works while they are still visible. Once they are covered up, proof becomes far harder.
Gather everything: quotes, messages, receipts, bank transfers, material lists. If you bought materials yourself, those documents too.
Two different problems
Non-performance. The work is not done. What is sought is completion, recovery of what was paid for work not done, or the cost of having someone else finish it.
Defective performance. The work is done, but badly. What is sought is rectification, a reduction in price, or the cost of putting it right.
The first step in both cases is a written demand with a specific deadline. It has practical value too: some contractors come back once they see the conversation has become written.
If it is a construction company rather than a tradesman
Then guarantee periods apply according to the type of work, and the other side is a company that can be checked. See developer or agent fraud.
If the property is yours but you are abroad
That is the usual position with holiday homes, and the cause of a great many of these disputes. Inspection and documentation are arranged through a representative.
A practical point for next time: do not pay the whole advance up front, and ask for stages tied to work actually completed.
What I will need
Everything in writing, messages included. The payment documents. Photographs before, during and now. A quote or specification if there was one. Details of the contractor.
This page provides general information and does not constitute legal advice on any specific case. Rights and deadlines depend on the facts and the applicable law.