A CP81 notice lets you know that the IRS has not received your tax return for a specific year. You need to check the year that the notice is referring to (it's in the upper right hand corner, and also in the body of the letter) and file the tax return for that year. The amount that they say they owe you will change once the return is processed. But if you are owed a refund you only have three years from the due date of the return to file the return and claim it.