recently added a new theme to it. After activating it I got this: Fatal error ...
Probably your theme doesn't like your version of WordPress.
Try switching to the unedited WP default Theme for a moment using the WP dashboard to rule out any theme-specific issue. If you don't have access to your admin area, you can switch to the default theme by renaming your current theme's folder in wp-content/themes and adding "-old" to the end of the folder name using via FTP or SFTP or whatever file management application your host provides. Alternately, you can remove other themes except the default theme. That will force your site to use it.