After the crisis has been resolved, the Ladybug Miraculous owner throws the Lucky Charm object they previously summoned into the air and yells "Miraculous" followed by their superhero name. This generates an explosion of pink energy that transforms into a massive swarm of ladybugs that fly around the damaged area and fix any and all damage done since the crisis began. This includes healing those who were hurt, fixing broken items whose destruction caused the crisis, undoing the effects of what the villain has done, any damage caused by the heroes themselves, and even repairing anomalies in the space-time continuum.
Besides fixing the damage of the past, with the help of Rabbit Miraculous superpower, Burrow, it can also fix the damage done in the future.
If Ladybug can't use her Lucky Charm object or doesn't have one, she can use her yo-yo to invoke it. While using Lucky Charm causes her to start losing power, using Miraculous Ladybug does not, it being an optional power she can use before she detransforms.
There are some conditions, however, that are not subject to the restoration power:
The magic ladybugs cannot revert anything on a mental and/or emotional state, unless the change in the mind and/or emotion was directly caused by the crisis such as done by akumatized villains.
The magic ladybugs cannot revive dead or deceased people.
Unless it was the result of a Miraculous or an akumatized villains powers, it can't undo demises that are apart of nature or done without said Miraculous or an akumatized villains powers.
The magic ladybugs cannot restore anything that has nothing to do with the recently solved crisis.
The magic ladybugs cannot restore a former guardian's memories when they pass their role, due to the Guardian's Code.
The magic ladybugs can only go as far as the damage range the akumatized or amokized champions have caused.
The magic ladybugs can only fix disasters that are caused by the akumatized or amokized champions, not pure natural disasters caused by the forces of nature.
The same as the Lucky Charm, this power can only fix the damage caused by the villain or sentimonster it was used against, but it seems this weakness can be broken when the holder retransforms, gets older or when two people or more are akumatized together.
The magic ladybugs cannot fix anything that's broken before an akumatized or amokized champion shows up.
↑November 2015 per Thomas Astruc in https://twitter.com/thomas_astruc/status/666886344602423296
Kartos: @Thomas_Astruc can Ladybug revert or heal injuries and death inflicted by akumas with her "fixing" power?
Astruc: @Kartos Anything
Kartos: @Thomas_Astruc :O I'm assuming the amount of time she can go back to "undo" is very limited?
Astruc: @Kartos limited to the apparition of the akumatized villain.