Xbox Live is currently suffering from some unspecified issues at the moment, which include preventing players from accessing games they've bought from the network digitally.
Xbox's official support twitter has tweeted that it is aware of the issue:
The proper teams are aware that users are unable to launch purchased content. Keep up with the investigation here: https://t.co/99xfLMWcZW
— Xbox Support (@XboxSupport) March 21, 2017
Microsoft hasn't explained what the issue is or when it will be resolved, only that you can keep an eye on the status of Xbox Live here.
We've reached out to Microsoft for comment and will update if we hear back.