What if I have not filed returns for ___ years ?
As a general matter, The IRS wants taxpayers to comply (file and pay) voluntarily. Voluntary does not mean that you do not have to pay taxes unless you want to. You must pay tax despite what tax protester organizations say.
While possible, criminal sanctions are normally not sought if you contact the IRS before they contact you regarding unfiled returns.
The IRS recognizes it is difficult to determine income tax liability from a number of years ago, e.g., records may be unavailable. Normally the IRS only requires you to file the last 6 years returns. If complete tax records are not available, a return based on the best information available can be filed, but this is a last resort and should be disclosed on the return.