Thank you for contacting me about your Tax issue. I will work hard to help you understand the issue clearly.
If you did not respond and you missed the deadline by a few days, then usually it's no problem. Send your proof in as soon as possible.
On the other hand, if you have received a letter of assessment of additional tax because you did not respond, then you will need to write to the Audit Reconsideration department and ask them to re-open the case. The address you will mail this request to is the same as the one you send your original information to, other than it is addressed to the reconsideration group