You rolled the damage off of it. If it had 2.7k of damage, and you rolled 2.3k damage to another stat, you are left with 400 damage.
Roll the damage back on the weapon, and (spoiler alert!) it will do damage again. Makes sense, right?
Please read: https://us.forums.blizzard.com/en/d3/t/things-that-are-not-bugs
- My Weapon Went from 2000+ DPS to 400
Go back to the mystic and enchant the +xxxx-xxxx element damage back onto the weapon and NEVER roll this affix off ever again (unless you want to be zdps). The +xxxx-xxxx elemental damage is a main component for the damage of the weapon.