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The Commissioner of Jurors in Nassau County would have to follow up the subpoena with a formal notice of non-compliance, to which you could either admit non-compliance and be penalized (usually a fine) or ask for a hearing on your non-compliance and present a reason. There could be all kinds of legitimate reasons for a person not to answer that questionnaire, so they don't just automatically find you in contempt.
You can see the law as to this here. Scroll down to "Section 128.12 Failure to respond to questionnaire or summons; procedure for noncompliance."
You are still in time to right this error. Just call the Commissioner of Jurors, tell him that you only just got your mail and discovered the questionnaire. You'd like to fill it in but you are a few days past the deadline. Ask if it's still okay to submit it. I'm sure they'll let you correct the oversight.
The contact information is on this page.
I don't know what the fine would be, but you still haven't gotten the formal notice of non-compliance, and you're not on trial and don't have to prove anything. Just tell him the truth, that this is the first notice you've received and you want to comply with the process and do your civic duty.