I deployed, but my live website did not change.

You may have created a Preview deployment rather than updating Production.

My Preview app has different data.

That may be intentional. Preview should often use isolated data.

Stripe worked in testing but stopped after launch.

Stripe Test Mode and Live Mode use separate settings and credentials.

The deployment succeeded, but the app is broken.

A successful deployment proves the version was deployed; it does not prove every workflow, database, payment, or integration works correctly.

Preview is using real customer data.

The environment mapping may be wrong and should be verified before continuing.