You need to go to the DMV and have a representative provide you a new copy of your permit. You can contact the DMV at 1-800-777-0133 or go to http://www.dmv.ca.gov to find the nearest office to you. You may go to any office and not necessarily the office in which you took your written test.
If you are under the age of 18 read the following carefully, it's IMPORTANT. Have the DMV representative indicate on your new learner's permit the issue date of your original learner's permit. Anyone under the age of 18 is required to have a learner's permit for at least 6 months before getting a drivers license. When the DMV reprints a learner's permit the issue date is changed to the day of the reprint. Having the DMV indicate the original issue date will ensure that your driver's license eligibility date remains the same.