Harry was unconcious, Harry was Voldermorts final Horcrux, so for Harry to be able to Kill Voldermort, Voldermort had to attempt to kill Harry, Voldermort destroyed his last Horcrux, which caused Harry to be unconsious due to the pain he was in. Harry then for some reason had a decsion whether to return to the real world or choose the path of death.
Hope that helped somehow