The penalties for not filing your return can be severe.  The IRS has a penalty for both not filing the return and not paying the tax.  Each of these penalties are 5% per month of the tax to a maximum of 25 %.  That is for both not filing and not paying.  This could amount to 50% of the total tax.  This is in addition to any criminal penalties that may be applicable.  You could be subject to one year in prison for each year not filed.

The IRS could file a Substitute Tax Return for you using the information they have available.  Typically, this would include any income reported to the IRS but no deductions except the standard deduction.  It is then up to you to file a correct return with the proper deductions.  No matter how late you file your tax return it is important that you file it for yourself and do not allow the IRS to file it for you.