How would you approach identifying the cause of a failure in a previously working request after some time has passed?

To identify the cause of a failure in a previously working query, I would sequentially perform the following steps:

1. Check for changes: find out if there have been any changes in the code, configurations, database, or environment since the last successful execution.
2. Logging and tracing: review application and database logs to find errors or warnings related to the query.
3. Reproduce the error: try to reproduce the failure in a test or local environment for more detailed analysis.
4. Data verification: ensure that the data the query works with has not changed and meets expectations.
5. Performance analysis: check for performance issues or locks in the database.
6. Use monitoring tools: apply APM systems or profilers to identify bottlenecks.

This systematic approach helps quickly localize and eliminate the cause of the failure.