I have been at a non-profit for 10 years and at my last review my supervisor told me that the agency has decided to cap my salary.

Unless there is a contractual provision saying otherwise (whether an individual employment agreement, or a collective bargaining agreement), and assuming that the employer is not disciminating on the basis of race, religion, gender, or some other statutorily prohibited basis, I am aware of no law which mandates that an employer must give raises, other than to keep up with minimum wage statutes.