If your loss was caused by a disaster in a federally declared disaster area, a special rule allows you to claim your loss on either your return for 2017 (the year the casualty event occurred) or on an amended return for 2016. This allows you to claim the deduction in the year when it does you the most tax-saving good. You cannot "carry forward" your loss to a later year (e.g. 2018) unless you are unable to use it all up in two years.
Included below is a link on how to amend your 2016 return:
https://ttlc.intuit.com/questions/3061514-how-do-i-amend-a-2016-return-in-turbotax
