From the Secretary of State’s website:
Notaries public who have lost their stamp or journal must send a letter immediately by certified mail to the Secretary of State explaining what happened to their stamp or journal and, if applicable, a photocopy of a police report. The letter should include your typed or printed name, notary public commission number, and your signature. If your journal was lost, include the time period of the journal entries and the expiration date of your commission in the letter. Upon written request, the Secretary of State will send a duplicate certificate of authorization so you can have a new stamp made. (California Government Code sections 8206 and 8207.3(e).) Please refer to Contact Information for the mailing address.