When i feel lack of energy to do my daily tasks, it becomes viable to slot work into smallest chunks. Let yourself do only several hours of work each day, then rest to recover.
For example, 1 hour timer to do any small piece of work, then 2 hours rest. The second day, 1 hour work, 2 hours rest, 1 hour work. Increase working slots with time iteratively, until you reach the desired productivity.
It won’t be possible to do 1-day comeback from the burnout state. It must be done incrementaly, adding small amount of work every day.
It’s no shame to rest, when your body require it. If you can allow it - rest as much time as needed. This allowed me to stay on productive track much-much longer, taking even 1 month of slow working.
Later, it will be a discipline - work at least X amount of hours every day, unrelated to your current mood. Morning procedures, lunch procedures. Everything according to the plan. But this is one of the hardest, and probably, most important skills a productive person can have.