Move: Physical activity (any movement of the body) is your first line of attack in the battle against burnout. You don't need an elaborate workout routine—just do something that gets you moving. It'll be worth it.
Talk to another human: We're wired for connection: we need it to not only thrive but to survive. This is why, when you don't feel like yourself, you should talk to someone.
Meditate—or just breathe: Try to be mindful, to focus on the present. Even if you last 90 seconds and can't get more than one or two breaths in without your mind wandering, it counts.
Indulge in some nostalgia: go back in time for a bit to take a sip of the same joy you felt when you first experienced the song/movie/show/book/whatever it is that you love most.