I don't know that anyone has ever been dead "for a few days". Within hours the decomposition process begins in the guts, blood coagulates in even less time, a whole host of proteins begin changing almost instantly. People have been technically dead for matters of minutes and have had profound experiences which I don't think can be readily dismissed, but the reason the dead can't come back to life is because the body is very quickly ruined as the inhabitation for any kind of a soul.