Cant play downloaded games

As DJ PY stated, you likely downloaded the demo, which you can find under Game Demos in your Game Library. The full game is not available for purchase on Games on Demand.

Since the core services of Live were acting up today, I’d try again when everything is normal.

Have you tried all the download troubleshooting steps?

If so I would suggest a power cycle.

All you have to do is, while the console is on, hold the power button for 10 seconds until turns off. Unplug the cord at all connection points for at least 15 seconds. Once that is done plug it in and power it up.

Did you try to reboot after you got the 'game failed to launch' message? If you get that same message after redownloading, try rebooting and then launching again.

You just downloaded the demo, because it is not available for a full download at the marketplace yet. You have to purchase it from any retail store.

That's the reason you why you cant play it.

Sorry