Site setting view is changed and cannot open site assets, site library and site pages

Hi all,

Help would be greatly appreciated.

1. When I view site settings now I can see a bunch of icons on the top and side. Screenshot attached. Is there any way to change it back to normal view.

 2. I have full control of the site but suddenly I cannot access some site contents such as Site Assets, Style Library, Site Pages. I get Something went wrong error. 

It used to work fine with no problem.

Any suggestions would be greatly appreciated.