Well, we know that heart transplants, for example, can give someone many more years to live. But if you transplant a good heart to a person who died 10 minutes ago in the same way you would to a living person, the person won't come back.
That's exactly how any heart transplants used to work: you stop the heart (now the patient is dead), cut it off, put the new one back. If the body is cold enough, 10 minutes might be OK.
The reason it does not work past 10 minutes without cooling is that brain cells start dying en masse, including ones running vital functions.
