Look at the determination where you were denied. I'm in AZ and don't know the specifics for CA, but on my form it says, "This determiantion becomes final unless a written appeal is filed by telephone, mail, by fax or in person with 15 calendar days after the mailing date shown at the top of this determination."

I'm hoping that yours maybe says "email or electronically." If nothing else, file your appeal. Your reason for being late is that it took CA 12 days to respond to your request. Had they responded immediately to your question, your appeal would have been timely. Also, I know that CA's phone system is still overwhelmed, so it's not unreasonable that you'd have tried to take the easy route and failed to mail or fax in your appeal request.

As big a pain as it is, when you get an adverse determination, appealling is generally the only way to get it fixed.