You have multiple possible remedies. I recommend that you first send a demand letter with a fixed date for payment. You may be entitled to 2% interest per month and recovery of your attorney's fees under the prompt payment statutes. Often a demand letter is sufficient to provoke payment. You can also record a mechanic's lien if the project is complete and the time for recording the lien has not yet passed. If there is a construction lender, you can serve a bonded stop payment notice within the same lien deadlines. And you can file a lawsuit to foreclose your lien, to enforce your stop payment notice, and to recover damages for breach of contract.