Here are a couple of things to check for:
The connection between Novi AMS and QuickBooks Online likely needs to be refreshed. Sometimes it's necessary to refresh your connection with QuickBooks Online for various reasons, much like you'd refresh an Internet browser window. It's much less daunting than it sounds. Visit your Association Settings inside of Novi and disconnect from QuickBooks Online. When Novi prompts you, re-connect to your QuickBooks account. This article will walk you through the process.
If the above does not work, it is possible that your QuickBooks Online subscription is suspended. Check your QuickBooks settings to ensure that your QuickBooks Subscription is still valid.