Are you flying an N-Reg aircraft?
Are you familiar with the re-registration process?
Are your name & address up to date for service of notice to renew?
See
http://www.lanepowell.com/wp-content/uploads/2010/07/0729102.pdf#page=2 for details. Written for Canadian clients but makes sense anyway. This link jumps to the table for renewal date but the whole article is worth a read.
John