If you have a W-2 from last year & your return was accepted/paid, you can file an amended return for that year (IE: If you have a W-2 from 2016 that wasn't reported, you can file an amended return for tax year 2016 reflecting this W-2), but you can not file last years W-2 on this years return (IE: If you have a W-2 from 2016 that wasn't reported, you can not report it on your 2017 return). You have 3 years to file an amended return, or 2 years from the tax date!
