Everything deleted after a backup

SOS!

I was making adjustments to an existing site based on a theme. I wanted to make a backup, but after I did everything the site became empty. All the information was saved in the Dashboard but pages and the theme are not available anymore.

I am not a professional, I was just asked to make some adjustments and I somehow ruined the whole website, so if you can explain what happened and how could I restore it I would be very thankful!