White screen

Hello!
The other day I was managing my WP page and when I changed theme, white screen appeared. How can I fix it? Is there something I can do in File manager on my server because I can’t even open the administrative page?

Here is link to my page: http://www.zvono.gimnazijang.hr/

Sorry for my bad english.

Thank you, hope you can help me!

The page I need help with: [log in to see the link]