If a game needs Steam, you will find the notice on the product page of each game
The game is now forever linked with your Steam account. If you need to reinstall the game, or if you wish to download and play it on another machine, please make sure you always use the same Steam account that you created when activating the game.
The game is now in your Steam library, just click Play!
Take a look at the Down detector page. This webpage is contstantly updated by users around the world, and so you can instantly see if the problem is with Steam or if it's with your computer.
For more information please contact Steam support
If you want to remove a game permanently from your Steam library take a look at that article here