Okay. The first thing you’ll want to do is restore the uncustomized Twenty-Fourteen theme. If the only file you re-wrote was the style.css file, you could download a fresh copy of the Twenty-Fourteen theme to your desktop and open up its style.css file, copy it and paste it into your site’s Twenty-Fourteen style.css file (replacing the one you overwrote). Alternatively, you could delete the Twenty-Fourteen theme from your themes folder and then just re-download a new copy of it to your themes folder and hit “activate,” but you may lose some settings.
At that point, you’ll have two options: either try to change your theme’s fonts via one of your plugins, OR create a child theme for Twenty-Fourteen and apply your font-family changes in the child theme’s style.css. You can learn to make a child theme here on the codex.
There is also a support forum specifically for the Twenty-Fourteen theme here, where you may find a thread that has already answered a similar question. Cheers!