I received a letter from the IRS indicating they did not get my tax payment that was submitted via your website. What should I do?

[Anonymous]

check the account the money was supposed to come out of ?  if it did, get a copy of the statement and send it with notice back to IRS.  circle amount and include letter that's states you paid on such and such a date and have enclosed the document showing payment.  

was the info for the account entered correctly?  did you have sufficient funds or credit line.  if not expect another notice  from IRS asking for penalties and interest

if the info was correct and there were sufficient funds submit notice, check for payment, written statement and whatever proof you can provide that shows the account info was correct.  in statement ask irs to abate any penalties or interest it might assess for late payment because you provided IRS with proper info and the failure to timely withdraw funds is an error on the part of the IRS.  this assumes you timely submitted return  and had payment scheduled for on or before 4/15/19