Menu and Header disappeard

Hi, I have already checked some possible sources. But when I change the theme the mistakes disappear. I cant see the menu and header of my website anymore. I am using jetpack. After reinstalling it worked well, but as soon as i work on the admin-panel the menu and the header disappears. i also deactived some plug ins. but it does not work. perhaps you have any idea.. as I a wrote: as soon as I change the theme, the mistake disappear. thank you www.priskaleutenegger.ch