Both are wrong You can only value something when it is lost. You can only value something when you loose it. OR You can only value something once you've lost it. There is a difference in tense between these three examples, but the main idea about when you can truly value something remains the same.

I think you mean lose. "Loose" is a different word entirely. To lose is to have something be lost or taken away. "Loose" means not tight. But as people said before, neither sentence is correct. You can only value something once it's lost. or You must lose something to know its value. You can say either of these and they mean the same thing, but usually the main difference is that tense. examples: I lost it yesterday. It is lost. Is it lost? *Lost is always past tense I might lose it. I will lose it. Did you lose it?

Time tense used, thats all as I see. Both describe same thing.