so when asgrad was into pieces and everyone left to go to earth for new asgrad, Loki then go back to old asgrad without telling Thor or anyone and pick Hela's unconscious body and took her to earth and took her to one and only friend of loki, your house and put hela in your guest room and said to take care of her and keep her a secret and left so Thor won't be suspicious

After a few hours later Hela wake up and look around and said "Where am I ? how am I alive?"