Please verify that your Product Advertising account is still active.
If you do not make any sales within a certain period of time, Amazon suspends the account and you have to re-apply.
If that is not the cause, then check to make sure your server has not changed (most specifically, that they have not modified the settings to allow CURL or fopen). Sometimes when a host updated a server configuration, the settings will change and that can cause an issue.
If you are getting sporadic results, then make sure the cache ahead is turned on in settings and lastly, try changing out your Access Keys.
Warm regards,
Don
