Hi @Andy ,
The main reason for this error is that the SharePoint server lost connection with the SQL database server due to network issues, so you need to check all the SharePoint server to database server connections.
- Open SQL Server Management Studio, check if you could open it well.
- Go to IIS Manager, check if the identify of the related application pool is correct and the application pool is started. Please restart the application pool.
- Do an IISRESET.
- Check your SQL Server services are up and running.

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