* Loss: This is the most common noun form and refers to the state of being lost or the act of losing something.
* Lost: This can be used as a noun in some contexts, particularly when referring to a specific instance of something being lost, such as "The lost treasure was finally found."
* Lost thing/item/property: This is a more specific way to refer to something that has been lost.
The best way to choose the right noun form is to consider the context of the sentence and what you are trying to express.