I installed the Lahanna Theme and used the documentation to do so. The menu item “Featured area” within the documentation tells you to go to “Appearance –> customize…” and to add main slider, link box or Intro widgets. I did this (added main slider), pressed “publish” and after that the whole blog structure in the frontend was gone.
Even the sample data was gone. The Page is empty and I have no clue what to do.
I can’t even switch between the frontend and backend because the specific header is also gone.
If I go back to appearance –> customize, I can’t even find the menu item “widgets” anymore.

Does somebody know what to do and can help?

I would appreciate.