How do you restore the functionality of a system or service after a failure?

To restore system or service operability after a failure, the following steps are usually performed:

1. Analyze the cause of the failure — study logs, metrics, and errors to understand what caused the problem.
2. Restart the service or components — often helps to bring the system back to working condition.
3. Rollback changes — if the failure was caused by recent updates, they can be reverted.
4. Restore from backups — if data is corrupted, restore it from backups.
5. Monitor after recovery — ensure the system operates stably.

It is important to have pre-prepared recovery procedures (runbooks) and automated tools for quick response.