That is because without doubt by your sister stepping on the laptop.
Your screen in now broken. They are very fragile and any exertion of excess pressure on them will result in a hair line crack of the screen that can sometimes not be seen by the naked eye or ribbons and controller boards of the LCD unit or screen to become loose, dislodged and broken.
If the laptop in not still under warranty or less than a year old check out what the returns procedure for repair and faults details.
If the repair and faults, most don`t do not cover accidental damage by you or a said person.
And with in warranty date cover.
Then you are going to have if you choose to attempt to open the laptop where the Lcd screen sits.
By using youtube for most...