Before we will help you to investigate the issue, you can try to repair it by yourself:
- At first please follow this article Troubleshoot problems with an app, one of suggested solutions is to try running the Apps troubleshooter.
- Make sure that you are using the latest available version. Just try to uninstall it and install again, this will force Windows to get latest available version from Windows Store. Also you can try to open Windows Store application, go to the application settings (WinKey+C) and choose App Updates. It will open page with button Check for Updates. Press it to see if there are some updates available for our application.
- We are unofficial Google Music client application and Google does not tell us when they change something in their API, but we are publishing fixes in one-two days.
- Take a look on our twitter feed if this is known issue https://twitter.com/outcoldplayer