"My key was lost."
Translation:Cái chìa khoá của tôi đã bị mất.
"Lạc" means you lose your way and cannot find the way (to the destination or back); e.g. I am lost in the forest - Tôi đang bị lạc trong khu rừng.
"Mất" means something can't be found (yet); this sentence is an example. "Mất tích" has a similar meaning but can be (and is usually) used for humans/animals.
(Don't use "mất" for humans because it is a euphemism for dead.)