A report that normally takes 2 seconds now takes over a minute. How would you identify the bottleneck and optimize the query?

I am preparing for SQL and database interviews and came across this real-world troubleshooting scenario. Suppose a report that usually executes in about 2 seconds suddenly starts taking more than a minute to complete.

I would like to understand:

What steps would you take to identify whether the bottleneck is caused by the SQL query, database indexes, server resources, or data growth?
How can tools such as EXPLAIN or EXPLAIN ANALYZE be used to identify performance issues?
What are the common causes of slow SQL queries, such as missing indexes, full table scans, inefficient JOINs, or outdated statistics?
How would you optimize the query without affecting data accuracy?
Which database monitoring tools or performance metrics should be checked during troubleshooting?
Could you share a real-world example of diagnosing and improving a slow-running SQL query?