After the crisis has been resolved, the Ladybug Miraculous owner throws the Lucky Charm object they previously summoned into the air and yells "Miraculous Ladybug!". 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, and even repairing anomalies in the space-time continuum. This power is even capable of bringing back the dead.
The Miraculous Ladybug doesn't only restore damage caused by the crisis, but also any damage caused by the heroes themselves.
Beside 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 yoyo 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:
↑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.