You should download your current /wp-content/themes/twentyfourteen/
directory using your file manager or FTP client.
https://codex.www.ads-software.com/FTP_Clients
What I’m suggesting is that you get your site back to working order with a clean version of Twenty Fourteen. Once that’s established, you add back in the things you may have done to your theme. I’m recommending that you retain your old Twenty Fourteen files to help get things back to the way they were, but I’m not suggesting you just dump the old back in with the new. That would get you back to where you are right now.
Additionally, I would also suggest that you create a child theme to work with the parent theme, Twenty Fourteen.
https://codex.www.ads-software.com/Child_Themes
This will aid you in developing your site, providing an opportunity to make changes that will allow you to fall back to a working version should something happen (like you’re currently experiencing).