I accidentally hid Terraria.

The unhide button isn't showing.

It should be at the bottom, on the right. (You might have to refresh the page.)

EDIT: Note that when I say "refresh", I'm just talking about hitting F5 to refresh the webpage in your browser; if that doesn't work, CTRL + F5 will reload everything on the page, not just data that's changed.
The GOG account refresh mentioned below is more of a "last-thing-to-try-before-contacting-GOG-Support" option than something you should try right away. It can be helpful in lots of situations, but it's also been known to occasionally mess up stuff that wasn't already broken, requiring you to contact Support anyway.

If the unhide button is not showing, try an account refresh:
https://secure.gog.com/account/refresh/

Do what HunchBluntley said, then the game should show up at the very bottom of your game shelf in a new section reserved for hidden games. Click on the game box of the hidden game and when it opens up you will find an "unhide" option near the bottom right of the content area for the game.