It is too late to amend your tax return for 2017. However, the casualty loss is limited to the amount over 10% of your income and you need to be able to benefit from itemizing your deductions to benefit from it. With the 2017 standard deduction of $6,350 (single) and $12,700 (joint filer) that you would take in lieu of the itemized deduction, the casualty loss may not have benefited you anyway.