Burnout is a very useful, if overused, phrase.
At best it indicates a low mood, a feeling of lethargy, exhaustion even but not in a physical way after a workout in the gym. It's more visceral than that . Strangely it's more difficult to spot until it's gone too far.
At its worst, it can mean the complete annihilation of any feeling or emotion to the point of not knowing yourslef or caring about anything. Words like quicksand spring to mind - the helplessness of being sucked under.