I'm not an attorney but I am a TX licensed insurance agent. My suggestion to you is to notify your E&O Insurance company and let them know about the subpoena. I know it sounds as if all they want is information from your notary journal, but this could transition into something else.
If you have E&O pull out a copy of the policy and you'll see a phone number for claims. If the company is CNA Surety/Western Surety I can give you the number. Tell the claims office about the subpoena and they will instruct you how to best respond.
If you don't have E&O Insurance...I really can't advise you what to do. If I were in your shoes I'd have to weigh the pros and cons of me admitting in writing that I don't have the journal.
Do you know if Texas required a journal back in 2010? Did you have E&O Insurance back in 2010?
Try your best not to keep the courts waiting. If you need more time let them know you need more time.