After defeating any villain, Ladybug throws the object she uses Lucky Charm into the air and yells "Miraculous Ladybug!". This generates an explosion of pink energy, eventually dispersing into small ladybugs that fly around Paris and fix any damage done since the appearance of the villain, including healing those who were hurt by an akumatized villain a seen in "Weredad", fixing Alix's broken Rabbit Miraculous and rejoining the two Ladybugs and Timebreakers created by time travel in "Timebreaker", and even reverting death.
As revealed in "Miraculer" and "Feast", that the Miraculous Ladybug can repair damage done by a sentimonster, no matter how long ago the damage was done.
It not only fixes the villain's damage, but can also fix the damage done by other Miraculous powers, such as Queen Bee's Venom, as well as Cat Noir's Cataclysm like with him being struck with his power in "Miraculer" As indicated in the third issue of Miraculous Adventures, the Miraculous Ladybug might be able clean up any damage, not just damage caused by a villain. Victory expect the Miraculous Ladybug can clean up their catastrophic damage in order to destroy the Trash Krakken, but Ladybug worries if it will be able to restore that much damage.
However, in some cases the Miraculous Ladybug can't fix or restore an object that doesn't have any remains left to fix, as in the cases of Santa Claus' hat in "A Christmas Special," Ella and Etta's hats in "Sapotis", Marinette's bracelet in "Gorizilla" and Cat Noir's rose in "Weredad". As seen in "Dark Cupid", the poster of Adrien which a mind-controlled Sabrina was vandalizing wasn't reverted by Ladybug's power, meaning that not all non-magical effects caused by an akumatized villain can be fixed by Miraculous Ladybug.
It seems that the power can repair damage done by users of the Ladybug Miraculous in "Desperada". It also can't restore purified sentimonsters seen in "Ladybug". Due to the code of the Guardians the power can't restore the memories of a former guardian when they pass their role like with Master Fu's memory in "Miracle Queen". Beside fixing the damage of the past and with the help of Bunnyx's burrow it can also fix the damage done in the future in "Cat Blanc".
If Ladybug can't use her Lucky Charm object or doesn't have one, she can just use her yoyo to invoke it. While using Lucky Charm causes Ladybug to start losing power, using Miraculous Ladybug does not, it being an optional power she can use before she detransforms.
↑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.