The IRS uses Notice CP516 to ask you to contact them immediately because your tax return is overdue. They are requesting tax returns from you because they think you owe. You also might receive a CP516 if you filed an extension but then later didn't file or if you had home mortgage interest but no income (the IRS wonders where you got the money).
You may have mailed the return but the IRS says they didn't get it. If you have the certified mailing receipt or overnight label then you can prove the filing date and there is a procedure for this. But, unless you have the mailing receipt, it is usually better to just re-file than argue. However, since you are filing an original return you have the opportunity to pour over your records and find more deductions!
However, some years may not require a return. If returns are due, then they must be carefully prepared with all advantage to you - the truth is the goal. Many people have lost some or all of their records. We are masters at finding evidence and lost documents. See Lost Documents.
This is NOT an audit & you will likely not be audited. But, the IRS believes you owe so you have to be careful & not rash.
Because of the legal and procedural implications of your response, it is not wise to rush out & file a return. We need to get the IRS file and create an Action Plan for your specific situation. Please follow the Prep Steps for IRS Notice CP516 & see Mr. Hopkins in Colorado Springs, Pueblo or Denver before proceeding!