I'm sorry you're dealing with this. Unfortunately, this pattern is common enough that there are well-established remedies. Let me outline your options.
1. CSLB Complaint: File a complaint with the California Contractors State License Board immediately. If this contractor is licensed, the CSLB has a Consumer Assistance fund that can help cover losses up to $25,000. If he's unlicensed, operating without a license is a misdemeanor in California (B&P Code 7028), and the CSLB investigates these cases. You can check his license status at cslb.ca.gov.
2. Criminal Complaint: What you're describing — taking money with no intention to complete the work — can constitute theft by false pretenses under Penal Code 532. Contact your local police department or DA's office. In many counties, the DA has a real estate fraud unit or consumer fraud unit that handles contractor fraud specifically.
3. Demand Letter: Even though he's ghosting you, send a formal demand letter to his last known address via certified mail. This creates a paper trail and gives him a deadline to respond. There are templates at /Demand-Letters/Home-Services-Construction/contractor-incomplete-work-demand-letters.html.
4. Deposit Limits: Here's something important — under California law (B&P Code 7159.5), a contractor cannot collect a deposit exceeding $1,000 or 10% of the contract price, whichever is less, for home improvement contracts. Your $12,000 deposit on a $24,000 contract massively exceeds this limit. That's an independent CSLB violation and strengthens your case.
5. Mechanics Liens: Check whether any subcontractors or material suppliers have filed or might file mechanics liens against your property. If the contractor hired subs and didn't pay them, they can lien your home. Get a preliminary title report to check.