There is a difference between not filing a tax return and filing your tax return after you complete your FAFSA. If you are completing your FAFSA early, then you should reasonably estimate your expected income.
According to FAFSA instructions, you should mark "Not going to file," if you have no income to report. Do not skip the "Have you completed a tax return" question. The same considerations apply if your parents will not file a return.
Only a select group of students do not have to answer any questions about their parents' income. For example, if your parents are in jail, you left home because of an abusive environment or if you do not have contact with your parents though you are not in foster/adoptive care, you should explain the situation and wait for your school's financial aid office to decide.
Special circumstances do not arise when your parents refuse to provide their tax return information or pay for your college. Also, you must still include their information even if you do not live with them or they do not claim you as a dependent.
If you have lost or misplaced you tax return, contact the IRS by telephone at (800) 829-1040 or by mail using IRS Form 4506T to request copies of past returns.