In your state (CA), if you are rejected due to a background check, the employer must disclose the reason in writing, including a copy of the check they relied on, and give you a chance to respond. Failing to do that, as they evidently had, or making the decision on some impermissible basis (e.g. on a dismissed criminal charge, or an arrest that did not lead to a charge and conviction) can subject to them a penalty which could be up to $10,000. Since they have not provided you the reason for the rejection, you wish to contact your state department of labor about filing a complaint--the state's help would be free.