500 GBs of data inaccessible on 365 for Nonprofits

Very sorry to hear this is happening. A better plan might have been to automatically roll everyone from the discontinued Business Premium grant onto the free Business Basic licence.

This page will provide you with some information about what happens after a subscription ends: What happens to my data and access when my Microsoft 365 for business subscription ends? | Microsoft Learn

You'll notice that admins retain access to the tenant and data for a while longer than regular users during the 'Disabled' state, before the data and tenant is deleted. So hopefully you can access as an admin and replace the expired licenses with the free Basic, or either of the paid licenses (Standard or Premium).

(Unfortunately, it won't be helpful to those who have already had their data deleted.)

Thanks
Matt