During setup when you first sign up for ManageByStats, you grant MBS developer access for the Amazon API.
This access can periodically timeout and need to be renewed. Renewal is quick and easy, but Amazon does not always let you know it's about to time out.
If the API access we need times out, you'll see a message like this on your Dashboard when you first log into ManageByStats.
You can also see it in your Settings > Seller Accounts:
If you see that message, check your Amazon Seller Central account and renew access. To do that, in Seller Central go to:
Settings > User Permissions
and then go to the bottom of that page, to the section titled:
Third party developer and apps
Click Visit Manage Your Apps:
Look for the ManageByStats entry and check the expiration info. If needed you can renew by clicking to Renew Access: