If the only income you had was Social Security, then you do not have to file a 1040.
This, from the Instructions for Form 1040:
Do not include any social security benefits unless (a) you are married filing a separate return and you lived with your spouse at any time in 2016 or (b) one-half of your social security benefits plus your other gross income and any tax-exempt interest is more than $25,000 ($32,000 if married filing jointly).
