Generally, if you are a non-exempt employee, your employer should pay you at an overtime rate for all hours worked in excess of 8 hours per day or 40 hours per week. If you are a non-exempt employee and you were not paid overtime pay that you were owed, you can sue your employer for that back pay up to three years, or possibly four years, after you worked the overtime hours.
California tends to have some of the most favorable employee rights laws in the country. Please note that I am not licensed there, but I am actively litigating several employee rights matters in CA via another firm's license - but I will not dispense CA legal advise. Under Federal Laws however, unless the employer has properly classified you as an exempt employee they are obligated to pay you overtime. There are different exemptions the employer might try to classify you as so its helpful to speak with a professional who can differentiate between the classifications and guide you as to your legal rights if you've been wrongfully deprived of overtime.
There are a couple of exceptions under which an employer can pay time and one half only for more than 80 hours in any two week period, but generally if one is a non exempt employee, i.e., not professional, executive, or administrative, he or she is entitled to overtime of time and one half for all work over 40 hours.
If you think you have been unfairly denied overtime pay, go to the U.S. Department of Labor office nearest you as well as the California parallel office. You can sue, but it will be faster and easier if the government acts on your behalf.