Omoi no Kakera -Close to-

Homura Motoki was walking down the street one day with his girlfriend Yuuna when they were struck by a car. When he woke up in the hospital, he realized something incredible had happened. He was having an out of body experience and no one seems to be able to see or hear him. Floating around in the air, Motoki set off to find Yuuna, only to discover she has lost all her memories of him because of the accident.

With only a few days left until his body can no longer be sustained, Motoki must now find a way to make Yuuna remember him so he can return back to his body.

Source: TheGamesDB