Welcome to the forums,
A lot may have happened to her and you will likely have better answers to your questions by directly asking your cousin.
Is your cousin experienced in taking care of pet birds? If not then lack of proper care has most likely led to your budgie's current condition, she may be sick in addition to possibly being traumatized, since you mention she's afraid of sounds.
Given the abrupt change on your budgie's behaviour and the fact that she is showing signs of lethargy (always sleeping), it would be best to have her seen by an avian vet specialist or at the very least an exotics vet or vet experienced with birds. I understand from your location that it may be more difficult to find a good professional.
For now and until you are able to find medical attention, you can keep your budgie warm by covering the cage on 3 sides and having some soothing music on low volume. This will hopefully decrease her stress levels and make her feel less jumpy.
If you have spray millet in hand, you can soak in electrolyte solution and if not you can add the solution to your budgie's drinking water.
That would hopefully give her a boost and open up her appetite, this link has detailed information: https://talkbudgies.com/your-budgies-...te-recipe.html
You can also offer her some egg food (by hard boiling an egg, mashing it all up and place it on a bowl) as this is highly nutritious and a good supplement to give to sick and convalescent birds.