You have a Compute Engine instance that you only use during business hours. To save costs, you stop the instance every evening and start it every morning.

What happens to the data on the instance's attached Persistent Disk when the instance is stopped?

When you stop a Compute Engine instance, the instance is shut down and you stop paying for CPU and memory. However, any attached Persistent Disks remain intact and retain their data. You continue to incur storage charges for the disk while the instance is stopped. When you start the instance again, it will boot up with all the data exactly as you left it. (Note: This is different from Local SSDs, where data is lost if the instance is stopped).