How can I get paid quickly, my wife is disabled and I am just a small business owner

I was hired to seal a basement wall from the exterior and interior, I have completed the work except for installing one piece of Sheetrock, the homeowner said he did not want me to finish the Sheetrock and asked me to leave the property, I have paid my employees and the work was done to the written Contract, job total was 3600.00 which I got 1800.00 down payment when They signed the Contract not that the work is finished he is knit picking and won't pay, What can I do to get the 1800.00 owed to me. Thanks John Spielberger