Assuming you are talking for another employee, I would recommend giving them either a Sabbatical or a very long vacation. If neither of those are options, I would recommend fundamentally changing their role so they are handling things in a completely different way.
However, it might be worth understanding why they are burnt out. Is your organization is set up so they have to be the "super hero" all the time? Are they not getting the recognition they deserve? Burn out happens, most of us have been there. Sometimes it is worth burning out employees for a short term goal, but you have to accept that there is a real long term cost to recover.
Note: If you are that burned out employee, you need to first recognize why you are burned out. Was it because of a big push, or is this structural in your organization? Then you need to ask, is it worth it to stay? If it truly is worth it, try to give yourself a "mental break". Either through a sabbatical/long vacation, or by doing a completely different job for awhile. If neither is possible, maybe it is time for a new opportunity.
