What steps should be taken if a web application suddenly stops responding or works very slowly?

If a web application stops responding or is very slow, the following steps should be taken:

1. Check the server status: CPU load, memory, disk, network.
2. Look at the application and server logs for errors or warnings.
3. Check the database: slow queries, locks.
4. Use monitoring tools (e.g., New Relic, Prometheus) to identify bottlenecks.
5. Check if the code or configuration has been recently changed.
6. Restart services if possible and safe.
7. If necessary, profile the application to identify "bottleneck" functions.

These steps will help quickly localize the problem and take measures to restore functionality.