Accidentally hid a game that I bought and can’t get it back
First things first: Any purchased app will not cost you (a second time). Even if you ordinarily search for it and click on BUY again. If you use the same credentials you used to buy it, the App Store will recognize it and will let you download it for free.

In order to unhide any app you once purchased, you need to sign into your Apple ID account inside iTunes, then click on your account name and choose "Account Info". If yo have hidden purchases, you have a section between "Apple ID Summary" and "Purchase History" called "iTunes in the Cloud", where you can manage hidden purchases.

Once in there, choose "Apps" on the right side at the top and then look for your inadvertently hidden app to "Unhide" it.

If you bought the app for OS X, the same can be done on the App Store for OS X.