Did you paste the file into your actual game files, then delete it? If you did, did it replace something and made you confirm the replacement? Either way will break your game.

Modfiles typically don't contain the entirety of the file they modify, only a section. The proper mod folder you're supposed to download and use Merges the modfiles with the gamefiles in a way that's easily removable. If you replace actual gamefiles, you'll lose like 90% of the file's contents, since you're only adding a tiny part of modified code. Never replace the actual game files, lots of people don't include all the file's contents simply because it's too much filesize.

If you did use proper modding installation, have you tried Verifying your Steam Cache? That usually fixes any problems with faulty files.