Website becomes blank after making changes in the theme

Hello,

I have two questions:

1: I am trying to make some changes to my website’s layout but whenever I do so, all website becomes white and no content appears. For example, if I am changing the page layout from standard to masonry, no content appears until I change it back to standard,  Also, the feature slider does not show.

2: Is it possible to add a slider like the attached images in the home page?

Thank you!