Well... that's why backups are essential - so you've followed the main cardinal rule !

Best of luck with future plugin installs - and by doing them in a test enviro first, you can't mess up a live site.

One more tip...

Whenever a CORE file is affected, re-name the ORIGINAL to xxxxxxxxxxxx.TEMP

Here's what I do...

Let's say I modify tpl_main_page.php

I rename it to tpl_main_page-php.TEMP

(notice how I leave the "php" in the name, but hyphenate it from the rest).

PS (I do not use "TEMP" as my file extension, but something else consisting of four letters which I do not reveal in public forums.)

Anyway... if things go belly up, then I remove my edited version of the core file, and name the original back to tpl_main_page.php