Fatal Error

– your theme “merchant” seems to be the issue.

– this theme is not in the WP official theme repository. Therefore, you’ll be more likely to find a good answer from your theme’s developers, for they have the access to its code.
http://codex.ww.wp.xz.cn/Forum_Welcome#Commercial_Products

– meanwhile, if you like, try switching to the unedited default Twenty Fourteen Theme for a moment using the WP dashboard to rule out any theme-specific issue. When 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.