What kind of 1099? 1099Misc, 1099R, 1099Div, etc. If it's for 2016 you have to amend your 2016 return to add it. Did you file 2016?
If it's for self employment income you don't need to report the individual 1099Misc. So if you already included the amount in your schedule C income you don't need to do anything.
