1. Lock your weapons and armor to prevent this in the future. 2. You can buy it again from the Exotic Blueprint. Shouldn't be hard to get it back to 310, especially now that the raid drops a ton of 310+ items. (Oh, you said Legendary. Well, legendary swords have no purpose other than to obtain an exotic version. You should be using a Raid Rocket or Machine gun for PvE if you're not using an exotic sword. All 3 of my legendary swords have sat in the vault ever since getting the exotic version.)