This may be caused by some action, such as changing the password, or revoking the granted permissions.
If no actions have been taken, then this may be due to the validity period of access.
The validity period of data access is 90 days after the user’s last session. After this period, you will be able to work in your BrandWizard personal account, but BrandWizard will not have access to Facebook account data. To restore access to the data, you need to re-authorize the permissions for BrandWizard.
If the permission has not been used through the personal account for 90 days, then you must provide it again. This rule applies even if the permission was approved through the BrandWizard verification and even if the user has been recently active.
You can read more in the Facebook documentation:
https://developers.facebook.com/docs/permissions/reference?locale=en_US
https://developers.facebook.com/docs/facebook-login/auth-vs-data/