Yes, you can appeal a rejected workers' compensation claim in California. If your claim is rejected by the insurer, you are entitled to dispute that determination by the state's Workers' Compensation Appeals Board (WCAB). The initial step is to submit an Application for Adjudication of Claim, which formally initiates your case. You might also be required to file a Declaration of Readiness to Proceed and request a hearing.
While the appeals process is underway, you can introduce evidence, such as medical records and witness statements, to substantiate your claim. A judge will consider the evidence and make a ruling. In the event the judge decides against you, appeals continue.
It's strongly advised to hire or consult with a workers' compensation lawyer. These matters are very intricate in terms of legal and medical issues, and having a lawyer substantially boosts your chances of getting a denial overturned and recovering the benefits that you're entitled to.