Someone who's lifeless has died. Your lifeless house plant probably got that way because you forgot to water it.
You can use the adjective lifeless to describe once-living things, like people, animals, and plants, and you can also use it for objects that appear inert: "The creepy thing about that painting is the clown's lifeless eyes." Things that are duller or droopier than they should be are also lifeless, like your lifeless hair or a writer's lifeless prose. Less common is using lifeless to mean "with no living things," like the lifeless planet Venus.