Think back through your career thus far. If you are a more recent graduate, include your schooling too. What is the hardest day you have faced? What challenge kept you up at night? What problem took the most amount of time to solve? The answers to these questions will help lead you to the toughest problem you have faced. Begin by sharing a high-level overview of the problem. The interviewer is most concerned with how you overcame the problem, so go more into depth with how you worked through the problem. Be sure to mention how long it took you to overcome the problem, and share that you never let the problem get the best of you. A positive attitude is key! Finally, mention what you learned from that experience and what you would do differently if a similar problem occurred again in the future.