Amazon has updated its API rates policy to reduce usage limits for accounts that are not driving enough affiliate revenue. API access can be temporarily revoked due to following two reasons:

1. The API call limit for each account is calculated based on revenue performance during the last 30 days. Access can be temporarily revoked if the API calls made during the day exceed the usage limit. The API will resume working the next day. The API limit increases as the account generates more affiliate revenue.

2. The account will temporarily lose access to its API if it has not generated referring sales using the API in the last 30 days. The good news is, you will regain access to Amazon API once your account again begins to drive referring sales again. 

More information available here.