call the number on the notice. there is the possibility the payor mailed it in and either the IRS lost it or has not yet processed it or maybe there was an error in the SSN the payor had on the 1099.  when you call, if they can't locate it ask how you should proceed.  they may want you to send them form 4852 along with a copy of the 1099R.  do ASAP because the IRS usually wants a response to the notice in 60 days.