Functions.php reverted to new one

First of all, sorry for not being able to phrase my question correctly.

Then here is my problem –
Some days ago I did some changes to my functions.php (one in wp_includes not in theme). I read that I should paste the code in function.php and did it without thinking in wp_includes functions.php…. so is it wrong to make changes in that functions.php ?

Because now, my functions.php (in wp_includes) all my changes have reverted, I don’t see the quick edit columns that I added from functions.php in my custom post anymore. This is really a problem for me since it took me long time to setup that code & populate fields etc. For changes, the only actual thing that makes me feel like could have caused this is my caching plugin reason being new functions.php consists some extra lines that setup cache filters or something. The plugin I use is WP Rocket but didn’t see any kind of such problems in their forums & I don’t want to post this there until I confirm I’m not at fault.

So if anyone has any idea about this, it would be really helpful. Getting back my old functions.php would be my life coming back to my body (not that you guys can do magic, but if I’m missing something important, that would be helpful.)

Thanks in advance,

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