If you haven't filed a tax return in a few years, you may still owe the IRS. You may have even been suprised when you received a Balance Due Notice in the mail from the IRS for years you haven't filed.
The reason for this is the SFR. An SFR stand for Substitute for Return. When you go more than 2 years without filing a return, the IRS might prepare a return for you. They will file you single with standard deductions. You will not get any other deductions that might be beneficial to you. Once they prepare a return for you and you owe tax, they will file that return and assess the tax against you. If they prepare a return for you and you are due a refund, they will not file that return. Thus, you will not get your refund. It is not in the best interest of the Treasury to hand out money on returns not filed by the tax payer.
Once your tax has been assessed, they will put you in the collections letter cycle.
Even though you have a SFR, you can still file a return on top of it with all of your deductions included and you correct filing status. This will file over the SFR and adjust your tax liability, but it will take months to process, so in the meantime, you need to be in a resolution with your supposed liability or the IRS will enforce collections (even if your return shows you don't owe). Doesn't seem fair but that's the way it is.
This is one of those areas where professional representation might be your best bet. It is very tricky when filing over SFRs and when filing old returns. If your returns don't match up to IRS records, you will be red-flagged for an audit for sure, delaying the whole process and even resulting in more penalties.