There could be a variety of reasons why your GA4 BigQuery export has stopped abruptly. Given your situation, here are a few things you could try:

1. Check the BigQuery data transfer logs: This could provide valuable insight into what went wrong. You can find this in BigQuery console under “Transfers.”

2. Validate the configuration: Make sure that the configuration has not changed. This includes the Google Analytics 4 (GA4) properties and data streams configuration in GA4 Interface, as well as GA4 data stream linkages in the BigQuery interface.

3. Manual export: Try manually exporting the data from GA4 to BigQuery for the project that’s causing issues. If it works, then there might be a problem in automated scheduling and not the export itself.

4. Check permissions: Ensure the account used to set up the export hasn’t had a change in permissions or has been disabled or deleted. If the user who started the export loses their access to the project, data export will be affected.

5. Check quotas and limits: Ensure you’re not hitting any quotas or limits that might affect your export process in BigQuery.

6. Reactivate the linkage: If all else fails, you might want to deactivate and reactivate the linkage between GA4 and BigQuery.

If these steps don’t work, it might be best to seek official Google Cloud Support assistance. However, they should provide you with enough to troubleshoot the issue.