I have just learned that my answer was due, but I did not file an answer. What do I do?

You must file a motion to the TTAB asking that the TTAB accept a late-filed answer. In your motion, you must to set forth the reasons why the filing is late. The TTAB will consider whether you have shown good cause to set aside the default. Do not forget to serve a copy on the other side, including a certificate of service, see TBMP § 113. You may also want to confirm that your mailing address is correct.