If you fail to file your tax return, the IRS may file a substitute tax return for you. Because substitute returns are filed in the best interest of the government, the only deductions you'll see are the standard deduction and one personal exemption
You will not get credit for deductions that you may be entitled to; such as exemptions for spouses and your children. You will not be given itemized deductions such as mortgage interest, charitable contributions, and property taxes you paid on your home. If you sold stocks or bonds you will be taxed on those sales proceeds as though your profit was 100% of the sales price. Regardless of what you have heard, you have the right to file your original tax return, no matter how late it's filed. If this has occurred you should immediately file an "original" return. Better late than never.